Arakhin 18A

ואחר כך מתה בתו, פקח ונתחרש וחזר ונתפקח, פתוח ונסתמא ואחר כך נתפתח, שפוי ונשתטה וחזר ונשתפה ⁠— כשר. זה הכלל: כל שתחילתו וסופו בכשרות ⁠— כשר.

and afterward his wife, who was the daughter of the father-in-law, died, which means that the witness is no longer related to the party involved; or when he was able to hear, and then became a deaf-mute, and again became able to hear; or when he could see, and subsequently became blind, and afterward could see again; or when he was halakhically competent, and then became an imbecile, and again became halakhically competent; in all these cases he is fit to testify.This is the principle: Any individual whose beginning and end is in a state of qualification to serve as a witness is qualified to testify, even if he was unfit in the interim. Evidently, it is not derived from the verse: “And he is a witness,” that the witness must be fit from the beginning to the end.

שאני התם, דאמר קרא: ״או ראה... אם לא יגיד״, בראייה והגדה תלא רחמנא מילתא, והא איכא. ואלא ״והוא״ למה לי?

The Talmud explains: The halakha is different there, with regard to testimony, as the verse states: “He is a witness, whether he has seen…if he does not utter” (Leviticus 5:1). This formulation indicates that God renders the matter of testimony dependent on seeing and recounting the content of his testimony. And in this instance there is both valid seeing and valid recounting, despite the fact that the witness was disqualified in the interim. The Talmud asks: But if so, why do I need the exclusion of the verse: “And he is a witness”?

לכדתניא: ראה סיאה של בני אדם עומדין, ועידיו ביניהן, ואמר ״משביעני עליכם אם יודעים אתם לי עדות שתבואו ותעידוני״, יכול יהו חייבין? תלמוד לומר ״והוא עד״, והרי לא ייחד עידיו.

The Talmud answers: The limiting clause is necessary for that which is taught in a baraita: If someone saw a crowd of people standing, and his witnesses were among them, and he said: I hereby administer an oath to you, if you know any testimony relating to me, that you will come and testify for me, one might have thought that this form of address suffices to single out the witnesses. This would mean that if the witnesses take a false oath that they do not know testimony with regard to the person who addressed them, they would be obligated to bring an offering of an oath of testimony. Therefore, the verse states: “And he is a witness,” to emphasize that the offering applies only to one who is singled out as a witness, and in this case the individual did not single out his witnesses, as he addressed a whole crowd of people. Consequently, the witnesses are exempt.