Related for Ketubot 50:3
כי ההוא דאתא לקמיה דרבי אמי א"ל מוחזקני בזה שהוא כהן א"ל מה ראית אמר ליה שקרא ראשון בבית הכנסת בחזקת שהוא כהן או בחזקת שהוא גדול שקרא אחריו לוי והעלהו ר' אמי לכהונה על פיו
Like that man who came before R. Ammi [and] said to him: I am convinced that he is a priest. He said to him: What have you seen? And he answered him: He read first in the synagogue. [He responded] As a priest or as a great man? After him a Levite read. And R. Ammi raised him to the priesthood on the strength of his testimony.
Tosefta Ketubot
Rabbi Lazar said: When [does this apply]? When there is a legal complaint in court [about the guy's status], but when there is no legal complaint in court, they raise to the priesthood on the basis of 1 witness. What constitutes a legal complaint? If they said: "How did Ploni jump into the priesthood?", "He never did the priestly blessing in shul", "He never took his priestly portion"—this is not a legal complaint. [If they said:] "He is son of a divorcée", "son of a halutzah", "a natin" or "a bastard"—this is a legal complaint. Rabbi raised [someone] to the priesthood based on 1 witness. Rabbi Yose raised [someone] to the priesthood based on 1 witness.
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