Abu al-Sahba' said to Ibn Abbas (may Allah be pleased with them both): Tell me from your heart, were three divorces not counted as one in the time of the Messenger of Allah (peace and blessings be upon him) and of Abu Bakr (may Allah be pleased with him)? He said: That was so. Then when the time of Umar (may Allah be pleased with him) came, people began giving divorces in succession, so Umar (may Allah be pleased with him) enforced all three divorces.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14974