Musnad Abu Ya'la al-Mawsili 152:190

Muhammad ibn Abi Bakr narrated to us, Khalid ibn al-Harith narrated to us, Shu'bah narrated to us, from al-Hajjaj, from his father, from a man among the companions of the Prophet, peace and blessings of Allah be upon him—I think it is Abdullah—from the Prophet, peace and blessings of Allah be upon him, who said: "When the heat intensifies, then delay the prayer, or delay [until the coolness of] the prayer."