Amr bin Hrm al-Azdy عمرو بن هرم الأزدي البصري · The Book of the Prayer

We were praying Zuhr with the Messenger of Allah at the time of intense heat (i.e., midday when the sun has just passed its zenith) and he said to us, "Wait for it to cool down before you pray, for intense heat is from the flaring up of the Hell-fire."