Musannaf Ibn Abi Shaybah

Ibn Aun reports that I asked Ibrahim: If a person begins the Witr prayer in the very last part of the night, what is its ruling? He replied: He should say the opening takbir and recite; when dawn breaks, he should pray one rak‘ah, then add another rak‘ah to it—these two rak‘ahs will become the two Sunnah rak‘ahs of Fajr. I mentioned this to Muhammad ibn Sirin, and he said: I do not know what this is?

Hazrat Ja'far bin Burqan says that I asked Hazrat Maymun, "If I recite a long surah in the night prayer and during this the morning arrives, and then it starts to get light, can I join another rak'ah with it and make this prayer the Sunnah of Fajr?" He replied, "Yes, you can make it so."

Hazrat Mujahid says that if a person wishes, he may begin prayer in the last part of the night and prolong its rak‘ah, so much so that when morning comes, he performs ruku‘, then adds another rak‘ah to it, and then counts these two rak‘ahs as the two sunnahs of Fajr.