Sahih Muslim Book 4, Hadith Number 1551.

Chapter : Excellence of observing two Rak’ahs in the mosque for one who comes back from a journey.

Mua’ada ‘Adawiyya reported ‘Aisha as saying: The Messenger of Allah (may peace be upon him) used to observe four rak’ahs in the forenoon prayer and he sometimes observed more as Allah pleased.