Genesis 25:1

Cetura, his third wife; the former two being perhaps both dead. This Abraham did in his 137th year, that God might have witnesses also among the Gentiles. Cetura was before one of his handmaids. (Menochius) --- God enabled him to have children at this advanced age; or perhaps, Moses may have related his marriage in this place, though it had taken place several years before. (St. Augustine, contra Jul. iii.) (Calmet) This learned father, City of God xvi. 34, supposes that the reason why Cetura is styled a concubine, though she was a lawful and only wife, is because her children prefigured heretics, who do not belong to the kingdom of Christ. (Worthington)