Musnad Abu Ya'la al-Mawsili 148:30

Abu Khaythama narrated to us, Waki' narrated to us, from Sufyan, from 'Abd al-Rahman ibn al-Asamm, who said: I heard Anas ibn Malik saying: The Messenger of Allah (peace and blessings of Allah be upon him), Abu Bakr, and 'Umar would not shorten the takbir.