Abu Amr al-Shaybani (may Allah have mercy on him) said that a man asked Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him): May a man who divorces his wife before consummating the marriage marry her mother? He said: Yes. He married her, and children were born. Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) asked Sayyiduna Umar (may Allah be pleased with him), and he said: Separate them. He said: But he has already had children by her. Sayyiduna Umar (may Allah be pleased with him) said: Even if ten children have been born, separate them all the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13905