Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 196
Narrated 'Aisha:
"When the prayers were first enjoined they were of two Rakat each. Later the prayer in a journey was kept as it was but the prayers for non-travellers 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."