Musannaf Ibn Abi Shayba 3:3983

Ibn Hisham narrated to us, from Ja’far ibn Burqan, who said: I said to Maymun, "I recite a long surah during the night, and the morning reaches me until it becomes very bright, so I add another [surah] to it and make it the two rak'ahs of the dawn [prayer]." He said, "Yes."