Musannaf Ibn Abi Shayba 27:402

'Affan narrated to us, he said: Sa'id ibn Zayd narrated to us, he said: 'Asim ibn Bahdala narrated to us, he said: Abu Wa'il narrated to us, from 'Aisha, she said: 'Uthman was writing the will of Abu Bakr. She said: Then he (Abu Bakr) fainted, so he (Uthman) hastened and wrote 'Umar ibn al-Khattab. When he (Abu Bakr) regained consciousness, he said to him: "Who did you write?" He said: "'Umar ibn al-Khattab." He said: "You wrote that which I intended to command you with, and if you had written yourself, you would have been worthy of it."