Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1479
Ibn 'Umar reported: The Messenger of Allah (may peace be upon him) said two rak'ahs at Mina, and Abu Bakr after him, and 'Umar after Abu Bakr, and 'Uthman at the beginning of his caliphate; then 'Uthman observed four rak'ahs, and when Ibn 'Umar prayed with the Imam, he said four rak'ahs, but when he observed prayer alone, he said two rak'ahs.