Sahih al-Bukhari 658
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · Two or more than two persons are considered as a group (for the congregational prayers)
Narrated Malik bin Huwairith:
حَدَّثَنَا مُسَدَّدٌ ، قَالَ حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ ، قَالَ حَدَّثَنَا خَالِدٌ ، عَنْ أَبِي قِلاَبَةَ ، عَنْ مَالِكِ بْنِ الْحُوَيْرِثِ ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا حَضَرَتِ الصَّلاَةُ فَأَذِّنَا وَأَقِيمَا، ثُمَّ لِيَؤُمَّكُمَا أَكْبَرُكُمَا " .
Prophet said (to two persons), "Whenever the prayer time becomes due, you should pronounce Adhan and then Iqama and the older of you should lead the prayer."