Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1459
'A'isha, the wife of the Messenger of Allah (may peace be upon him), said Allah prescribed the prayer as two rak'ahs, then it was completed (to four rak'ahs) at the place of residence, but was retained in the same position in journey as it was first made obligatory.