Sa'id bin al-Musayyab (may Allah have mercy on him) narrates from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that four things have been locked (made binding): 1. vows, 2. divorce, 3. emancipation, and 4. marriage.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14994