Musannaf Ibn Abi Shayba 3:4325

Abu Khalid narrated to us, he said: Abu Usama narrated to us, he said: Ubaydullah ibn Umar narrated to us, from Nafi’, from Ibn Umar, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Make the end of your prayer at night Witr."