Musannaf Ibn Abi Shayba 3:4390

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from Yahya ibn Abi Kathir, from Abu Nadra, from Abu Sa’id, from the Prophet, peace and blessings of Allah be upon him, that he said: "Perform the Witr prayer before you reach the morning."