Musnad Ahmad 921
It was narrated from ‘Ali (رضي الله عنه) that He said on the day of the battle of the Camel:
The Messenger of Allah (ﷺ) did not promise us anything on which we might base a claim of succession, rather it is something that we thought of ourselves, then Abu Bakr was appointed as his successor; may the mercy of Allah be upon Abu Bakr, for he did well and remained steadfast. Then ‘Umar was appointed as his successor, may the mercy of Allah be upon ‘Umar, for he did well and remained steadfast, until the religion of Islam became well established.
حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَنْبَأَنَا سُفْيَانُ، عَنِ الْأَسْوَدِ بْنِ قَيْسٍ، عَنْ رَجُلٍ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ أَنَّهُ قَالَ يَوْمَ الْجَمَلِ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَمْ يَعْهَدْ إِلَيْنَا عَهْدًا نَأْخُذُ بِهِ فِي الْإِمَارَةِ وَلَكِنَّهُ شَيْءٌ رَأَيْنَاهُ مِنْ قِبَلِ أَنْفُسِنَا ثُمَّ اسْتُخْلِفَ أَبُو بَكْرٍ رَحْمَةُ اللَّهِ عَلَى أَبِي بَكْرٍ فَأَقَامَ وَاسْتَقَامَ ثُمَّ اسْتُخْلِفَ عُمَرُ رَحْمَةُ اللَّهِ عَلَى عُمَرَ فَأَقَامَ وَاسْتَقَامَ حَتَّى ضَرَبَ الدِّينُ بِجِرَانِهِ‏.‏