Musannaf Ibn Abi Shayba 3:4470

Abu Bakr narrated to us, he said: Ibn Mubarak narrated to us, from Abd al-Hakim, from Sa'id ibn al-Musayyib, who said: The Messenger of Allah (may Allah bless him and grant him peace) established the Witr prayer just as he established the Fitr (Eid al-Fitr) and the Adha (Eid al-Adha).