(18) CHAPTER. If there are many travellers, Adhan and Iqama should be pronounced, (the same is to be observed) in Arafat and Al-Muzdalifa too.
Narrated Malik bin Huwairith: Two men came to the Prophet with the intention of a journey. The Prophet said, When (both of) you set out, pronounce Adhan and then Iqama and the oldest of you should lead the prayer.
Authentic