When Moshe ascended heaven to receive the commandments there is a tradition that Hashem changed his appearance to that of someone else. Why was he changed and to whom was he changed? Is there a source for this tradition, I can't find it.
According to the Midrash Rabbah (28:1), the angels wanted to attack Moshe for coming to take the Torah down to earth. Hashem altered his face to resemble Avraham's and said to them, "Aren't you ashamed to attack the person who was so hospitable to you?"
Avraham served the angels cream, milk, and veal (Bereishit 18:8). To commemorate this meal, which contributed to the Jewish people receiving the Torah on Shavuot, we eat a dairy meal, and a meat meal an hour later. (See Shelah, Shavuot 180b: Sha'arei Halachah Uminhag, vol. 3, p. 38.)
Or in a slightly different version, "hey angels, you're not such hot stuff yourself, you had milk and meat when you visited this guy, remember?"
Gemara Shabbos 88b-89a has several similar stories of G-d & Moshe defending against the angel's claims, though not this one.