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’.*
*The name of a house in Madina.
Bukhari transmitted it.