Musannaf Ibn Abi Shayba 2:148

Abu Usama narrated to us, from Abd al-Malik, from Ata, who was asked about travelers: Do they perform the Adhan and the Iqamah? He said: The Iqamah suffices them, unless they are scattered, and he wishes to gather them, then he performs the Adhan and the Iqamah.