Muwatta Malik, Book 22, Hadith 1025 (In-Book)

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to do kaffara for a broken oath by feeding ten poor people. Each person got a mudd of wheat. He sometimes freed a slave if he had repeated the oath.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يُكَفِّرُ عَنْ يَمِينِهِ، بِإِطْعَامِ عَشَرَةِ مَسَاكِينَ لِكُلِّ مِسْكِينٍ مُدٌّ مِنْ حِنْطَةٍ وَكَانَ يَعْتِقُ الْمِرَارَ إِذَا وَكَّدَ الْيَمِينَ ‏.‏

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also referenced under: Book 22, Hadith 13 (USC-MSA)