Musnad Ahmad 32:544

Harun ibn Ma'ruf narrated to us, he said: Ibn Wahb narrated to us, he said: Ibn Abi Dhi'b narrated to me, from Mawhub ibn 'Abd al-Rahman ibn Azhar, from Anas ibn Malik: That he used to differ from 'Umar ibn 'Abd al-'Aziz. So 'Umar said to him: "What compels you to do this?" He said: "I saw the Messenger of Allah, peace and blessings of Allah be upon him, performing a prayer. Whenever you conform to it, I pray with you, and whenever you differ from it, I pray and return to my family."