Musnad Ahmad 30:355

Yazid narrated to us, Ibn Abi Dhi'b informed us, from Abu al-Walid and 'Abd al-Rahman ibn Sa'd, both of them from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, the intensity of heat is from the scorching breath of Hell. So, when the heat intensifies, then delay the prayer until it cools."