Musnad Ahmad 790:175

Hassan ibn Musa and Affan narrated to us, they said: Abu Hilal informed us—Affan said: Qatadah informed us, and Hassan said: from Qatadah, from Abu Hassan al-A’raj, from Imran ibn Husayn, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, would speak to us for the majority of his night about the Children of Israel; he would not stand up except for the greatness of the prayer," meaning the prescribed, obligatory one. Affan said: "For the majority of his night, he would speak to us about the Children of Israel; he would not stand up except for the greatness of the prayer."