Musnad Ahmad 31:684

'Affan narrated to us, Aban narrated to us, Yahya ibn Abi Kathir narrated to us, he said: Abu Nadra narrated to me, that Abu Sa'id al-Khudri narrated to him, that the Messenger of Allah, peace and blessings of Allah be upon him, was asked about the Witr, and he said: "Perform the Witr before the dawn."