Musannaf Ibn Abi Shayba 26:438

Jarir narrated to us, from Mansur, from Ibrahim, who said: Umar summoned a slave woman and asked her, "Which of the two is he [the father of the child]?" She said, "I do not know; they both had intercourse with me during the same period of purity." So Umar divided him [the child] between them.