Musannaf Ibn Abi Shayba 3:1482

Waki’ narrated to us, he said: Al-Mukhtar ibn ‘Amr al-Azdi narrated to us, he said: I asked Jabir ibn Zayd about prayer while traveling. He said: "If you pray alone, then pray two rak'ahs. And if you pray in a congregation, then pray with their prayer."