The Book of Leading the Prayer (Al-Imamah)
10/4. Chapter:
Those who are older going forward (to lead the prayer)
Sunan an-Nasai
أَخْبَرَنَا حَاجِبُ بْنُ سُلَيْمَانَ الْمَنْبِجِيُّ، عَنْ وَكِيعٍ، عَنْ سُفْيَانَ، عَنْ خَالِدٍ الْحَذَّاءِ، عَنْ أَبِي قِلاَبَةَ، عَنْ مَالِكِ بْنِ الْحُوَيْرِثِ، قَالَ أَتَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم أَنَا وَابْنُ عَمٍّ لِي - وَقَالَ مَرَّةً أَنَا وَصَاحِبٌ لِي - فَقَالَ " إِذَا سَافَرْتُمَا فَأَذِّنَا وَأَقِيمَا وَلْيَؤُمَّكُمَا أَكْبَرُكُمَا " .
It was narrated that Malik bin Al-Huwairith said: "I came to the Messenger of Allah (ﷺ) with a cousin of mine" - once he said, "with a friend of mine" - and he said: 'When you travel, call the Adhan and Iqamah, and let the older of you lead the prayer.'"
Reference : Sunan an-Nasa'i 781In-book reference : Book 10, Hadith 5English translation : Vol. 1, Book 10, Hadith 782