Musnad Ahmad 557
It was narrated that Abu Wa`il said:
I said to ‘AbdurRahman bin ‘Awf: How could you swear allegiance to ‘Uthman (رضي الله عنه)  and not to `Ali (رضي الله عنه)? He said: It is not my fault. I started with `Ali and said: I swear allegiance to you in accordance with the Book of Allah, the Sunnah of His Messenger, and the way of Abu Bakr and ʼUmar (رضي الله عنهما) . He said: As much as I can. Then I offered it to `Uthman (رضي الله عنه) and he accepted it.
حَدَّثَنَا عَبْد اللَّهِ، حَدَّثَنِي سُفْيَانُ بْنُ وَكِيعٍ، حَدَّثَنِي قَبِيصَةُ، عَنْ أَبِي بَكْرِ بْنِ عَيَّاشٍ، عَنْ عَاصِمٍ، عَنْ أَبِي وَائِلٍ، قَالَ قُلْتُ لِعَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ كَيْفَ بَايَعْتُمْ عُثْمَانَ وَتَرَكْتُمْ عَلِيًّا رَضِيَ اللَّهُ عَنْهُ قَالَ مَا ذَنْبِي قَدْ بَدَأْتُ بِعَلِيٍّ فَقُلْتُ أُبَايِعُكَ عَلَى كِتَابِ اللَّهِ وَسُنَّةِ رَسُولِهِ وَسِيرَةِ أَبِي بَكْرٍ وَعُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ فَقَالَ فِيمَا اسْتَطَعْتُ قَالَ ثُمَّ عَرَضْتُهَا عَلَى عُثْمَانَ رَضِيَ اللَّهُ عَنْهُ فَقَبِلَهَا‏.‏