Muwatta Malik, Book 22, Hadith 1 (USC-MSA)

Yahya related to me from Malik from Ibn Shihab from Ubaydullah ibn Abdullah ibn Utba ibn Masud from Abdullah ibn Abbas that Sad ibn Ubada questioned the Messenger of Allah, may Allah bless him and grant him peace, and said,"My mother died while she still had a vow which she had not fulfilled." The Messenger of Allah, may Allah bless him and grant him peace, said, "Fulfill it for her."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، أَنَّ سَعْدَ بْنَ عُبَادَةَ، اسْتَفْتَى رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ إِنَّ أُمِّي مَاتَتْ وَعَلَيْهَا نَذْرٌ وَلَمْ تَقْضِهِ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ اقْضِهِ عَنْهَا ‏" ‏ ‏.‏

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also referenced under: Book 22, Hadith 1012 (In-Book)