Musnad Ahmad 31:76

Qutaybah narrated to us, Ya'qub—meaning al-Qari—narrated to us, Suhayl narrated to us, from his father, from Abu Sa'id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Do not sell gold for gold, nor silver for silver, except weight for weight, like for like, equal for equal." And he 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."