Reference for Kiddushin 149:21
א"ל אביי מי דמי התם גר ישן וממזרת חדשה אמרי בר ישראל הוא דקא נסיב ממזרת הכא אידי ואידי כי הדדי נינהו אלא כי אתא רב דימי אמר רבי אלעזר סבר לה כרבי ישמעאל
Hence both are necessary. It was taught: And thus did R'Eleazar say: A Cuthean may not marry a Cuthean. What is the reason? - Said R'Joseph: He was treated as a proselyte after ten generations. For it was taught: A proselyte, until ten generations, may marry a mamzereth; thereafter he is forbidden [to marry] a mamzereth. Others state: [He is permitted] until the name of heathenism has completely fallen away from him. Said Abaye to him: How compare! There it is a proselyte of ancient [stock] and a recent mamzereth, so it will be said: He is an Israelite marrying a mamzereth,' whereas here they are both alike? - But when R'Dimi came,<span class="x" onmousemove="('comment',' V. p. 46, n. 6.');"><sup>28</sup></span> he said: R'Eleazar agrees with R'Ishmael,