During the time of the Messenger of Allah ﷺ, the time of Abu Bakr, and the first two years of Umar's caliphate, pronouncing divorce three times at once was counted as a single divorce. Then Umar bin Khattab said, "The people have become hasty in a matter in which they were previously given time to reflect. Perhaps we should hold them to it." So he made it binding upon them.
كان الطلاق على عهد رسول الله ﷺ وأبي بكر وسنتين من خلافة عمر، طلاق الثلاث واحدة، فقال عمر بن الخطاب: إن الناس قد استعجلوا في أمر قد كانت لهم فيه أناة، فلو أمضيناه عليهم فأمضاه عليهم.
Muslim narrated it in the Book of Divorce (1472) via Abdul-Razzaq, who reported to us, Maamar informed us, from Ibn Tawus, from his father, from Ibn Abbas, and he related it.In Ibn Abbas's statement there is an indication of the consensus of the companions during the time of Abu Bakr (which was two years and a fraction) and two years of Umar's caliphate, that the triple pronouncement counted as one. Then Umar bin Khattab put into effect a ruling making the three count as three. But the community did not unite upon this. Those among the companions who opposed him and gave rulings contrary to his were Zubayr bin Awwam, Abdul-Rahman bin Awf, Ali bin Abu Talib, and Ibn Masud. Among the successors were Ikrima and Tawus, and after them Muhammad bin Ishaq, some of the companions of Malik, some of the companions of Abu Hanifa, and some of the companions of Ahmad. As for those who came after them, they are too numerous to count, among them Sheikh Islam bin Taymiya, his student Ibn Qayyim, and others.