Musannaf Ibn Abi Shayba 35:315

Ibn al-Mubarak narrated to us, from Abd al-Karim, from Sa'id ibn al-Musayyib, who said: The Prophet, 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).