Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1477
Salim b. 'Abdullah (b. 'Umar) reported on the authority of his father that Allah's Messenger (way peace be upon him) observed the prayer of a traveller, i. e. two rak'ahs in Mina, and other places; so did Abu Bakr and 'Umar, and 'Uthman too observed two rak'abs at the beginning of his caliphate, but he then completed four.