Musnad Abu Ya'la al-Mawsili 244:14

Abu Musa Ishaq ibn Ibrahim al-Harawi narrated to us, Yazid ibn Harun narrated to us, Sa'id ibn Abi 'Aruba informed us, from Qatada, from Raja' ibn Haywa, from Qabisa ibn Dhu'ayb, from 'Amr ibn al-'As, who said: Do not confuse for us the Sunnah of our Prophet, may Allah bless him and grant him peace. The waiting period ('idda) of the umm al-walad (a slave woman who has borne a child to her master), when her master dies, is four months and ten days.