Musannaf Ibn Abi Shayba 22:319

Yazid ibn Harun narrated to us, he said: Ibn 'Awn informed us, from Muhammad, who said: When Abu Bakr was made caliph, Ali stayed in his house. It was said to Abu Bakr [about this], so he sent for him [asking], "Did you dislike my caliphate?" He said, "No, I did not dislike your caliphate, but the Quran was being added to. When the Messenger of Allah (peace and blessings of Allah be upon him) passed away, I took an oath upon myself that I would not put on my cloak except for prayer, until I had gathered it [the Quran] for the people." Abu Bakr said, "You have seen well."