It is narrated from Sayyiduna Malik bin Abdullah al-Khuza'i (may Allah be pleased with him); he said: "I went on campaign with the Messenger of Allah (peace and blessings be upon him), and I never saw an imam whose prayer was lighter than that of the Messenger of Allah (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16700