Musannaf Ibn Abi Shayba 3:5817

Yazid ibn Harun narrated to us, from Habib, from Amr ibn Haram, who said: Jabir ibn Yazid was asked about people who depart from their city to the wilderness: "Do they pray two [rak'ahs] as long as they are nomads, until they return to their city?" He said: "They do not complete the prayer in the vicinity as long as they are nomads."