Musnad Abu Ya'la al-Mawsili 152:202

Al-Akhnasi Ahmad ibn Imran narrated to us, Muhammad ibn Fudayl narrated to us, and I heard him saying: Ibrahim al-Hajari narrated to us, from Abu al-Ahwas, from Abdullah, from the Messenger of Allah (may Allah bless him and grant him peace), who said: "Indeed, Allah is Witr (Odd/One) and He loves the Witr. So, when you clean yourself with stones (istijmar), then make it an odd number."