Musnad Ahmad 1153:190

Abu Mu'awiyah narrated to us, Abd al-Rahman ibn Abi Bakr al-Qurashi narrated to us, from Ibn Abi Mulaykah, from Aisha, who said: When the Messenger of Allah, peace and blessings of Allah be upon him, became heavy [with illness], the Messenger of Allah, peace and blessings of Allah be upon him, said to Abd al-Rahman ibn Abi Bakr: "Bring me a shoulder blade or a tablet so that I may write a document for Abu Bakr, that no one may disagree over him." When Abd al-Rahman went to stand up, he said: "Allah and the believers refuse that there should be any disagreement over you, O Abu Bakr."