Ubaydah narrates from Sayyiduna Ali (may Allah be pleased with him) that he said: My opinion and that of Sayyiduna Umar (may Allah be pleased with him) were united on freeing the ummahat al-awlad, but later I held that they should be kept as slaves. (Ubaydah said:) I said: "Your opinion together with that of Sayyiduna Umar (may Allah be pleased with him) is dearer to me than your opinion alone."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21794