Musnad Ahmad 27:595

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Hayyan—meaning al-Bariqi—who said: It was said to Ibn 'Umar, "Our Imam lengthens the prayer." Ibn 'Umar said: "Two rak'ahs of the prayer of the Messenger of Allah (may Allah bless him and grant him peace) were lighter than, or like, one rak'ah of the prayer of this man."