Musnad Ahmad 551:3

Yazid ibn Harun narrated to us, he said: Sa'id informed us, from Qatadah, from Raja' ibn Haywah, from Qabisah ibn Dhu'ayb, from 'Amr ibn al-'As, who said: "Do not confuse for us the Sunnah of our Prophet regarding the waiting period ('iddah) of the umm al-walad (a slave woman who has borne a child to her master). When her master dies, it is four months and ten days."