Musnad Abu Ya'la al-Mawsili 49:8

Al-Qawariri narrated to us, Affan bin Muslim narrated to us, Wahb narrated to us, Abdullah bin Uthman bin Khuthaym narrated to us, from Nafi’ bin Sarjis, from Abu Waqid al-Laythi, that he heard him saying: "Indeed, the Messenger of Allah (may Allah bless him and grant him peace) was the lightest of people in prayer, and the most constant of them upon himself."