Sahih Bukhari

Narrated `Aisha:""When the prayers were first enjoined they were of two rak`at each. Later the prayer in a journey was kept as it was but the prayers for non-travelers were completed."" Az-Zuhri said, ""I asked `Urwa what made Aisha pray the full prayers (in journey)."" He replied, ""She did the same as `Uthman did