Musannaf Ibn Abi Shayba 2:142

Waki’ narrated to us, from Sufyan, from Khalid, from Abu Qilabah, from Malik ibn al-Huwayrith, who said: I came to the Prophet, may Allah bless him and grant him peace, and with me was a cousin of mine. He said: "When you two travel, then give the adhan and the iqamah, and let the older of you lead you in prayer."