Ibrahim said: Qadi Shurayh (may Allah have mercy on him) would hardly ever go back on a judgement he had given, until al-Aswad bin Yazid (may Allah have mercy on him) narrated to him from Sayyiduna Umar (may Allah be pleased with him) that he said, concerning a free woman who is married to a slave and bears him children, and then their father is freed: their wala passes to those who freed their father. So Qadi Shurayh (may Allah have mercy on him) adopted it.
This chain is sahih (authentic), and it is possible that al-Aswad narrated it to him from both Umar and Ibn Mas'ud, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الولاء / 21525