Yahya related to me from Malik from Salih ibn Kaysan from Urwaibn az-Zubayr that A'isha, the wife of the Prophet, may Allah blesshim and grant him peace, said, "The prayer was prescribed as tworakas, both when settled and when travelling. Then the travellingprayer was kept as it was, and an increase was made in the prayer whensettled. "
USC-MSA web (English) reference: Book 9, Hadith 9
Arabic reference: Book 9, Hadith 337