Babylonian Talmud, tractate Shabbat, page 56B:
Rabbi Yehudah taught in the name of [Rabbi] Shmuel:
In the hour that [King] Solomon married the daughter of Pharaoh [who had converted to Judaism], the angel Gabriel created Rome [which later persecuted Israel brutally].

Babylonian Talmud, tractate Yebamot, page 24B:
No converts to Judaism will be accepted in the era of the Messiah.

Babylonian Talmud, tractate Yebamot, page 24B:
No converts to Judaism were accepted in the era of King David and King Solomon

Babylonian Talmud, tractate Yebamot, page 109B:
Evil after evil will come upon those who accept converts.

Babylonian Talmud, tractate Yebamot, page 109B:
Rabbi Chelbo taught: Converts are as difficult for Israel as a skin disease.

Babylonian Talmud, tractate Kiddushin, page 70B:
Rabbi Chelbo taught:
Converts are harmful to Israel as sapachat [a kind of supernatural leprosy].
RASHI explains: They are not careful with mitzvot and those who are near them regularly are drawn after them and learn from their deeds.

Babylonian Talmud, tractate Bechorot, page 30B:
If a Gentile is prepared to accept the Torah, except for ONE religious law, we do NOT accept him. Rabbi Yosi ben Judah says: Even if he rejects one of the laws of the Sages.
NOTE: Tosfeta, tractate Demai 2:6 replaces the word Gentile with the word convert.

Babylonian Talmud, tractate Niddah, page 13B:
Our Rabbis taught: Converts and Meshachkin BeTinokot delay the arrival of the messiah.

Minor Tractates of the Talmud, Tractate Gerim, chapter 1, law 7:
Anyone who converts to Judaism for the sake of [marrying] a Jewish woman, or fear, or love [of money] is NOT a convert…
Anyone who is NOT converted from purely religious motives is NOT a convert.

Midrash Tanchuma, Parshat Emor, chapter 11:
The Golden Calf was built by converts.

Yalkut Shimoni, comment on Shemot, chapter 12, remez 213:
When a man converts to Judaism because he wants to marry a Jewish woman, the Holy One Blessed Be He says to him:
You converted because of a nebelah [the carcass of an animal that is not kosher because it was not slaughtered correctly].

Rabbi Avraham Yitzchak HaKohen Kook, Daas Cohen on Yoreh Deah #154:
“Therefore it is wonderful what you and the other holy rabbis [of Argentina] have done in making and strengthening the boundaries in Argentina where there is a great breakdown in the walls and there is a great number of gerim who are not sincere – and you have decided not to accept gerim at all.