Yevamot 47b:5יבמות מ״ז ב:ה׳
The Gemara analyzes the baraita. The Master said in the baraita: With regard to a potential convert who comes to a court in order to convert, the judges of the court say to him: What did you see that motivated you to come to convert? And they inform him of some of the lenient mitzvot and some of the stringent mitzvot. The Gemara asks: What is the reason to say this to him? It is so that if he is going to withdraw from the conversion process, let him withdraw already at this stage. He should not be convinced to continue, as Rabbi Ḥelbo said: Converts are as harmful to the Jewish people as a leprous scab [sappaḥat] on the skin, as it is written: “And the convert shall join himself with them, and they shall cleave [venispeḥu] to the house of Jacob” (Isaiah 14:1). This alludes to the fact that the cleaving of the convert to the Jewish people is like a scab.
אָמַר מָר: גֵּר שֶׁבָּא לְהִתְגַּיֵּיר, אוֹמְרִים לוֹ: מָה רָאִיתָ שֶׁבָּאתָ לְהִתְגַּיֵּיר? וּמוֹדִיעִים אוֹתוֹ מִקְצָת מִצְוֹת קַלּוֹת וּמִקְצָת מִצְוֹת חֲמוּרוֹת. מַאי טַעְמָא? דְּאִי פָּרֵישׁ — נִפְרוֹשׁ. דְּאָמַר רַבִּי חֶלְבּוֹ: קָשִׁים גֵּרִים לְיִשְׂרָאֵל כְּסַפַּחַת, דִּכְתִיב: ״וְנִלְוָה הַגֵּר עֲלֵיהֶם וְנִסְפְּחוּ עַל בֵּית יַעֲקֹב״.