Sahih Al Bukhari Hadith Number 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.

18  Shortening the Prayers (At-Taqseer)

Hadith number 1082
Volume no 2
Chapter: 2 ( As-Salat (the prayers) at Mina (during Hajj)