Musannaf Ibn Abi Shayba 3:3984

Yazid narrated to us, from Jarir ibn Hazim, from Ya'la ibn Hakim, that Mujahid said: If a man wishes, he may begin a rak'ah at the end of the night, lengthening it until, when morning arrives, he performs the ruku' (bowing). Then, he joins another (rak'ah) to it, and counts them as the two rak'ahs of the dawn (Fajr).