Sahih Bukhari Volume 002, Book 020, Hadith Number 196.

Narated By '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."