Sahih Muslim Book 4, Hadith Number 1458.

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

‘Aisha, the wife of the Messenger of Allah (may peace be upon him), reported: The prayer was prescribed as two rak’ahs, two rak’ahs both in journey and at the place of residence. The prayer while travelling remained as it was (originally prescribed), but an addition was made in the prayer (observed) at the place of residence.