Musnad Ahmad 1077:9

Abdullah ibn al-Harith narrated to us, he said: Yunus read it to me, from Ibn Shihab, he said: Abdullah ibn Tha'labah—and the Messenger of Allah (peace and blessings of Allah be upon him) had wiped his face—informed me: "That he saw Sa'd ibn Abi Waqqas performing Witr with a single Rak'ah, not adding to it until he stood in the middle of the night."