Sa'id bin al-Musayyab and Abu Bakr bin Abd al-Rahman (may Allah have mercy on them both) said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: When four months pass, it is one divorce, but the husband has more right to take her back as long as she is in her waiting period ('iddah).
This is how Muhammad bin Ishaq bin Yasar narrated it from al-Zuhri. Imam Malik bin Anas (may Allah have mercy on him) contradicted him and narrated it from al-Zuhri from Sa'id and Abu Bakr as their own statement, not attributed to Umar (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإيلاء / 15223