Musannaf Ibn Abi Shaybah 6514 — A man who begins praying at night

Mujahid said: If a person wishes, he may begin a prayer at the end of the night and lengthen its rak'ah, until when morning comes he bows; then he joins another rak'ah to it, and then counts these two rak'ahs as the two sunnah rak'ahs of Fajr.