Musnad Ahmad 926
It was narrated from al-Musayyab bin ‘Abd Khair that  his father said :
‘Ali (رضي الله عنه) stood up and said: The best of this ummah after its Prophet are Abu Bakr and ‘Umar, then we got involved in matters which Allah will judge as He wills.
حَدَّثَنَا عَبْد اللَّهِ، حَدَّثَنِي وَهْبُ بْنُ بَقِيَّةَ الْوَاسِطِيُّ، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، عَنْ حُصَيْنٍ، عَنِ الْمُسَيَّبِ بْنِ عَبْدِ خَيْرٍ، عَنْ أَبِيهِ، قَالَ قَامَ عَلِيٌّ فَقَالَ خَيْرُ هَذِهِ الْأُمَّةِ بَعْدَ نَبِيِّهَا أَبُو بَكْرٍ وَعُمَرُ وَإِنَّا قَدْ أَحْدَثْنَا بَعْدَهُمْ أَحْدَاثًا يَقْضِي اللَّهُ تَعَالَى فِيهَا مَا شَاءَ‏.‏