Sahih Muslim Book 4, Hadith Number 1477.

Chapter : Compensation of the missed prayer and excellence of observing it promptly.

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’ahs at the beginning of his caliphate, but he then completed four.