Muwatta Malik #337

Yahya related to me from Malik from Salih ibn Kaysan from Urwa ibn az-Zubayr that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "The prayer was prescribed as two rakas, both when settled and when travelling. Then the travelling prayer was kept as it was, and an increase was made in the prayer when settled. "