Musannaf Ibn Abi Shayba 13:1453

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hanzala, from al-Qasim ibn Muhammad, who said: Ibn Abbas was asked about a man who became Muslim while having captive women: "May he have intercourse with them before they have completed their waiting period (istibra')?" He said: "No."