Narrated `Aisha: Originally, two rak`at were prescribed in every prayer. When the Prophet migrated (to Medina) four rak`at were enjoined, while the journey prayer remained unchanged(i.e. two rak`at).
Hadith ReferenceSahih Bukhari / The Merits of Al-Ansar / Hadith: 3935
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة