Musannaf Ibn Abi Shayba 3:907

Ali ibn Mushir narrated to us, from Yazid ibn Abd al-Rahman ibn Sabit, who said: Abu Mahdhura gave the adhan for the Dhuhr prayer in Makkah. Umar said to him, "Is it your voice, O Abu Mahdhura, that I heard?" He said, "Yes, I have saved it for you, O Commander of the Faithful, so that I may let you hear it." Umar said, "O Abu Mahdhura, you are in a land of intense heat, so delay the prayer until it is cooler, then delay it until it is cooler."