Sahih al-Bukhari Book 10, Hadith 27
Two men came to the Prophet (ﷺ) with the intention of a journey. The Prophet (ﷺ) said, "When (both of) you
set out, pronounce Adhan and then Iqama and the oldest of you should lead the prayer."
We were in the company of the Prophet (ﷺ) on a journey and the Mu'adh-dhin wanted to pronounce the Adhan for the (Zuhr) prayer. The Prophet (ﷺ) said to him, "Let it become cooler." Then he again wanted to pronounce the Adhan but the Prophet; said to him, "Let it become cooler." The Mu'adh-dhin again wanted to pronounce the Adhan for the prayer but the Prophet (ﷺ) said, "Let it become cooler," till the shadows of the hillocks become equal to their sizes. The Prophet (ﷺ) added, "The severity of the heat is from the raging of Hell."
We came to the Prophet (ﷺ) and stayed with him for twenty days and nights. We were all young and of about the same age. The Prophet (ﷺ) was very kind and merciful. When he realized our longing for our families, he asked about our homes and the people there and we told him. Then he asked us to go back to our families and stay with them and teach them (the religion) and to order them to do good things. He also mentioned some other things which I have (remembered or ) forgotten. The Prophet (ﷺ) then added, "Pray as you have seen me praying and when it is the time for the prayer one of you should pronounce the Adhan and the oldest of you should lead the prayer.