Rabbi Shlomo Yitzchaki (Rashi) (1040-1105) identifies the Ancient of Days with God (the Holy One) in his commentary on Daniel 7.
Rabbi Shlomo Yitzchaki (Rashi), Commentary on Daniel 7:9, Sefaria, accessed Septemebr 6, 2022
די כרסון רמיו. כסאות הוטלו ונתקנו לישב במשפט אחד לדין וא' לצדקה לישראל:
until thrones were set up Thrones were set up and established to sit in judgment, one for judgment for the nations of the world and one for charity for Israel.
ועתיק יומין יתיב. הקב"ה יושב למשפט על המלכות הזו ועל שלפניה שהכעיסו לפניו והציקו לבניו:
and the Ancient of Days sat The Holy One, blessed be He, was sitting in judgment on this kingdom and on those before it, who provoked Him and oppressed His children.
כתלג חיור. ללבן עונות עמו:
as white as snow to whiten the iniquities of His people.
ושער ראשה כעמר נקא. מנקה עצמו מזכיות שיש לעכו"ם לפניו ומשלם להם כל שכרם בעולם הזה בויקרא רבה:
and the hair of His head was like clean wool He cleanses Himself of the merits that the nations have before Him, and He pays them all their reward in this world. In Lev. Rabbah.