Musannaf Ibn Abi Shayba 3:3979

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Mis’ar narrated to us, from a sheikh from the Ansar, he said: Mis’ar said: I think he is Uthman, from Sa’id ibn Jubayr, he said: The Prophet, peace and blessings of Allah be upon him, would sometimes lengthen the two rak'ahs of the dawn prayer.