Anas said that when it was very cold the Prophet observed the prayer early, and when it was very hot he delayed the prayer till it was cooler. The reference is to the Friday prayer.
As-Sa’ib b. Yazid said that in the time of God’s Messenger, Abu Bakr and ‘Umar the beginning of the summons on Friday was when the imam took his seat on the pupilt, but in 'Uthman’s time when (the people were numerous he added the third call on az-Zaura’.*