"I came to the Prophet ﷺ with a cousin of mine" - on an another occasion he said: "with a companion of mine" - "and he said: 'When the two of you travel, call the Adhan and the Iqamah, and let the older of you lead the prayer.'" (Using translation from Nasāʾī 634)  

أَتَيْتُ رَسُولَ اللهِ ﷺ أَنَا وَابْنُ عَمٍّ لِي وَقَالَ مَرَّةً أُخْرَى أَنَا وَصَاحِبٌ لِي فَقَالَ «إِذَا سَافَرْتُمَا فَأَذِّنَا وَأَقِيمَا وَلْيَؤُمَّكُمَا أَكْبَرُكُمَا»