Musannaf Ibn Abi Shayba 3:4468

Yazid ibn Harun narrated to us, from Sufyan ibn Husayn, from al-Zuhri, from Ata ibn Yazid al-Laythi, from Abu Ayyub, who said: The Messenger of Allah (may Allah bless him and grant him peace) said to me: "Perform Witr with five. If you are unable, then with three. If you are unable, then with one. If you are unable, then gesture with a gesture."