Musannaf Ibn Abi Shayba 13:1544

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Isma’il and Zakariyya, from al-Sha’bi, from Shurayh: He said: ‘Umar entrusted to me that I should not authorize the gift of a slave girl until she has moved into her house for a year or she gives birth to a child. Isma’il said: I said to al-Sha’bi, "Do you see if she remains unmarried, is it permissible?" He said, "Yes."