Muwatta Malik, Book 29, Hadith 1256 (In-Book) ⓘ
Malik related to me from Nafi that Abdullah ibn Umar said, "The idda of an umm walad when her master dies is one menstrual period."
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ قَالَ عِدَّةُ أُمِّ الْوَلَدِ إِذَا تُوُفِّيَ عَنْهَا سَيِّدُهَا حَيْضَةٌ .
also referenced under: Book 29, Hadith 92 (USC-MSA)