Musnad Ahmad 30:2826

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah, may Allah bless him and grant him peace, said: "When the heat becomes intense, then delay the prayer until it cools, for the intensity of the heat is from the scorching of Hell."