Sahih al-Bukhari 1082
Narrated `Abdullah bin `Umar:
I offered the prayer with the Prophet, Abu Bakr and `Umar at Mina and it was of two rak`at. `Uthman 
in the early days of his caliphate did the same, but later on he started praying the full prayer.
حَدَّثَنَا مُسَدَّدٌ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ أَخْبَرَنِي نَافِعٌ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ صَلَّيْتُ مَعَ النَّبِيِّ صلى الله عليه وسلم بِمِنًى رَكْعَتَيْنِ، وَأَبِي بَكْرٍ وَعُمَرَ، وَمَعَ عُثْمَانَ صَدْرًا مِنْ إِمَارَتِهِ ثُمَّ أَتَمَّهَا‏.‏