The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 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.
Source materials are from the University of Southern California MSA site|
Hadith eBooks converted from Imaan Star