Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Salah (Prayers)

said that Allah's Messenger (SAW) was on a journey and Sayyidina Bilal (RA) was with him too. He intended to call the iqamah for the Salah of Zuhr, but the Prophet (SAW) said, "Let It cool down!" Then he again Intended to call..Allah's Messenger (SAW) said, "Let it be cooler for the Zuhr", till they saw the shadows of hillocks and called the iqamah and offered the Salah of Zuhr. Then Allah's Messenger (SAW) said, "The extreme heat is the severity of Hell. So, observe the Salah of Zuhr when it is cooler." [Ahmed 21434, Bukhari 539, Muslim 616, Abu Dawud 407, Ibn e Majah 683]