Sahih al-Bukhari 1082
Sahih al-Bukhari · Book 18: Shortening the Prayers (At-Taqseer) · As-Salat (the prayers) at Mina (during Hajj)
Narrated `Abdullah bin `Umar:
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.
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.