Musnad Ahmad 498:2

Hajjaj narrated to us, he said: I heard Shu'bah, he said: I heard 'Abd Rabbih ibn Sa'id narrating from Anas ibn Abi Anas—from the people of Egypt—from 'Abdullah ibn Nafi' ibn al-'Amya', from 'Abdullah ibn al-Harith, from al-Muttalib, from the Prophet, may Allah bless him and grant him peace, that he said: "The prayer is two by two." And he mentioned the like of it.