Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1460
'A'isha reported: The prayer was prescribed as consisting of two rak'abs, the prayer in travelling remained the same, but the prayer at the place of residence was completed. (Zuhri said he asked 'Urwa why 'A'isha said prayer in the complete form during journey, and he replied that she interpreted the matter herself as 'Uthman did.)