Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(35)
Chapter: Two or more than two persons are considered as a group (for the congregational prayers)
(35)
باب اثْنَانِ فَمَا فَوْقَهُمَا جَمَاعَةٌ
Sahih al-Bukhari 658

Narrated Malik bin Huwairith:

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."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 658
In-book reference : Book 10, Hadith 52
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 627
  (deprecated numbering scheme)