Mishkat al-Masabih 4 Prayer كتاب الصلاة 114

وَعَن مَالك بن الْحُوَيْرِث قَالَ: أَتَيْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَا وَابْنُ عَمٍّ لِي فَقَالَ: «إِذَا سَافَرْتُمَا فأذنا وأقيما وليؤمكما أكبركما» . رَوَاهُ البُخَارِيّ

Malik b. al-Huwairith said that he and a cousin of his came to the Prophet, who said, “When you two are on a journey, you should call the adhan and repeat the iqama , then the older of you should act as imam .” This is how Bukhari transmitted it.