Pirkei Avot 5:20משנה אבות ה׳:כ׳
Judah ben Tema said: Be strong as a leopard, and swift as an eagle, and fleet as a gazelle, and brave as a lion, to do the will of your Father who is in heaven. He used to say: the arrogant is headed for Gehinnom and the blushing for the garden of Eden. May it be the will, O Lord our God, that your city be rebuilt speedily in our days and set our portion in the studying of your Torah.
Mesillat Yesharim 5:25מסילת ישרים ה׳:כ״ה
The third detrimental factor is [evil] company, namely, the company of fools and sinners. This is what scriptures says: "he who befriends the fools will be broken" (Prov.13:20). We can see many times, even after the truth of a man's duty for divine service and watchfulness of it has been established by him, he becomes lax in it or transgresses certain commandments so that his friends do not mock him or in order to be able to mingle freely in their company.
Mesillat Yesharim 5:28מסילת ישרים ה׳:כ״ח
If he happens to find himself in the company of those who mock him, he should not give heart to this mockery. On the contrary, let him mock them and shame them. Let him consider in his heart - if he had an opportunity to profit a great amount of money, would he leave what he needed to do for this due to other people's mocking him? How much more so, to not want to lose his soul for the sake of sparing himself some mockery.
Mesillat Yesharim 5:29מסילת ישרים ה׳:כ״ט
In this manner the sages of blessed memory warned us: "be brazen as a leopard... to do the will of your Father in Heaven" (Avot 5:20). And David said: "I will speak of Your testimonies before kings, and will not be ashamed" (Ps. 119:46).
Mesillat Yesharim 6:6מסילת ישרים ו׳:ו׳
You can observe that man's nature weighs very heavily upon him. For the earthiness of the physical is gross. Therefore a man does not want to exert himself and labor. But he who wants to merit to the service of the Creator must strengthen himself against his own nature, mustering strength and zeal.
Mesillat Yesharim 6:7מסילת ישרים ו׳:ז׳
If he leaves himself in the hands of his [natural] heaviness, it is a certainty that he will not succeed. This is what the Tana (Mishnaic sage) stated: "be brazen as a leopard, light as an eagle, swift as a deer, and mighty as a lion to do the will of your Father in Heaven" (Avot 5:20). Likewise, the Sages counted among the things which need strengthening: "Torah and good deeds" (Berachot 32b). This is explicitly stated in scripture: "be strong and very courageous [to observe and do all of the Torah...]" (Yehoshua 1:6). For great strengthening is needed for one who wants to bend his nature to its opposite.
Mesillat Yesharim 20:17מסילת ישרים כ׳:י״ז
Behold, a man is obligated to guard all of the mitzvot in all of their fine details, doing so before any person, whoever it may be, and not be afraid or ashamed before him. Likewise, it is written: "I will also speak of Your testimonies before kings, and will not be ashamed" (Tehilim 119:46). And we also learned "be brazen as a leopard..." (Avot 5:2).
Mesillat Yesharim 20:18מסילת ישרים כ׳:י״ח
But, here too one needs to discern and make distinctions, for all this was said regarding the mitzvot themselves which we are completely obligated in. In those one should "set his face like flint" (Isaiah 50:7).
Mesillat Yesharim 20:19מסילת ישרים כ׳:י״ט
But there are some additional matters of Piety, which if a person were to do before the common people, they will laugh at him and ridicule him, thereby sinning and incurring punishment through him, and this is something he could have abstained from doing since these things are not complete obligations. Thus, for such things, it is certainly more proper for the Chasid to abstain from it than to do it. This is what scripture says: "and walk discreetly with your G-d" (Michah 6:8). Many great Chasidim abstained from their pious practices when in the presence of the common masses because it appears like arrogance.
Mesillat Yesharim 7:12מסילת ישרים ז׳:י״ב
On this our sages exhorted us numerous times. They said: "one who begins a Mitzva but does not complete it buries his wife and sons" (Bereishis Raba 85:3).
Sotah 13b:6סוטה י״ג ב:ו׳
Rabbi Ḥama, son of Rabbi Ḥanina, says: Anyone who performs a matter but does not complete it, and then another comes and completes it, the verse ascribes credit to the one who completed it as if he had actually performed the entire act. Due to the fact that the children of Israel completed Joseph’s burial, the Torah ascribes them credit as if they had performed the entire act.
Sotah 13b:7סוטה י״ג ב:ז׳
Rabbi Elazar says with regard to one who initiates performance of a mitzva but does not complete it when capable of doing so: He is also demoted [moridin] from his position of greatness, as it is written: “And it came to pass at that time, that Judah went down [vayyered] from his brethren, and turned in to a certain Adullamite, whose name was Hirah” (Genesis 38:1). Usage of the term “went down” indicates that the rest of Judah’s brothers had demoted him from his position of greatness because he began the process of saving Joseph, but he did not complete it.
Sotah 13b:8סוטה י״ג ב:ח׳
Rabbi Shmuel bar Naḥmani says: The episode with regard to Judah also indicates that one who initiates performance of a mitzva but does not complete it will also bury his wife and children as Judah did, as it is written: “And in process of time Shua’s daughter, the wife of Judah, died” (Genesis 38:12), and it is written further: “And the sons of Judah: Er, and Onan, and Shelah, and Perez, and Zerah; but Er and Onan died in the land of Canaan” (Genesis 46:12).
Mesillat Yesharim 9:4מסילת ישרים ט׳:ד׳
A man needs to realize that he was not placed in this world for tranquility but rather for toil and exertion. He must conduct himself only as the laborers who perform work for their employers similar to what was said "we are day laborers" (Eruvin 65a) and in the manner of soldiers in their war formations who eat swiftly and sleep irregularly, always ready to engage in battle.
Mesillat Yesharim 9:5מסילת ישרים ט׳:ה׳
On this scripture says "man was born to toil" (Iyov 5:7). When a man habituates himself in this manner, he will certainly find the service of G-d easy since he does not lack the preparation and readiness for it. Along these lines our sages of blessed memory said "this is the way of Torah, eat bread with salt, drink water in small measure, and sleep on the ground" (Avot 6:4) which is a general statement of distancing to the utmost extreme from the comforts and pleasures.
Mesillat Yesharim 9:6מסילת ישרים ט׳:ו׳
Another factor which diminishes Zeal is great fear and apprehension of what the future may bring so that sometimes one may fear the cold or the heat. Other times mishaps or illnesses. Other times from the wind and so on and so forth. This is what Shlomo, peace be unto him, said: "The lazy one says there is a lion in the way; a lion is among the streets." (Mishlei 26:13).
Mesillat Yesharim 9:7מסילת ישרים ט׳:ז׳
The sages already denigrated this trait and attributed it to the sinners, with many verses to support them such as "the sinners in Zion are afraid; trembling has seized the flatterers." (Isaiah 33:14). One of the great sages upon noticing his disciple afraid said to him: "you are a sinner" (Berachot 60a).
Berakhot 60a:17ברכות ס׳ א:י״ז
The Gemara relates: This student was once walking after Rabbi Yishmael, son of Rabbi Yosei, in the marketplace of Zion. Rabbi Yishmael saw that the student was afraid. He said to him: You are a sinner, as it is written: “The transgressors in Zion are afraid, trembling has seized the ungodly” (Isaiah 33:14). The student replied: And is it not written: “Happy is the man that fears always” (Proverbs 28:14)? Rabbi Yishmael said to him: That verse is written with regard to matters of Torah, that one should be afraid lest he forget them. For everything else, one must trust in God.