During the time of the Messenger of Allah ﷺ, the time of Abu Bakr, and the first two years of Umar bin Khattab's caliphate, pronouncing divorce three times at once was treated as a single divorce. Then Umar said, "People have grown hasty in a matter in which they were once given time to reflect. Perhaps we should hold them to it." So he made it binding upon them.
حدثنا عبد الرزاق حدثنا معمر عن ابن طاوس عن أبيه
كان الطلاق على عهد رسول الله ﷺ وأبي بكر وسنتين من خلافة عمر بن الخطاب طلاق الثلاث واحدة فقال عمر إن الناس قد استعجلوا في أمر كان لهم فيه أناة فلو أمضيناه عليهم فأمضاه عليهم
Its isnad is sahih. Muslim (1:423–424) and Hakim (2:196) both narrated it through Abdul-Razzaq, and Hakim said, "Sahih according to the criteria of the two sheikhs, but they did not include it," and Dhahabi agreed with him. But it should be noted against both of them that it is in fact in Sahih Muslim.I have given this hadith a full explanation in my book Nizam al-Talaq fil-Islam, page 42 and following. See also 2387.