Musnad Abu Ya'la al-Mawsili 1:26

Abu Khaythama narrated to us, Yahya ibn Hammad narrated to us, Abu 'Awana narrated to us, from Sulayman al-A'mash, from Isma'il ibn Raja', from 'Umayr, the freed slave of al-'Abbas, from Ibn 'Abbas, who said: When the Messenger of Allah (may Allah bless him and grant him peace) passed away and Abu Bakr was made successor, al-'Abbas disputed with 'Ali regarding things the Messenger of Allah (may Allah bless him and grant him peace) had left behind, [taking the matter] to Abu Bakr. Abu Bakr said: "Something that the Messenger of Allah (may Allah bless him and grant him peace) left behind and did not move, I will not move."