Muwatta Malik 1016

These are the words of a Companion, not of the Prophet ﷺ. The chain is graded on its own terms.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ، أَنَّهُ سَمِعَهُ يَقُولُ أَتَتِ امْرَأَةٌ إِلَى عَبْدِ اللَّهِ بْنِ عَبَّاسٍ فَقَالَتْ إِنِّي نَذَرْتُ أَنْ أَنْحَرَ ابْنِي ‏.‏ فَقَالَ ابْنُ عَبَّاسٍ لاَ تَنْحَرِي ابْنَكِ وَكَفِّرِي عَنْ يَمِينِكِ، ‏.‏ فَقَالَ شَيْخٌ عِنْدَ ابْنِ عَبَّاسٍ وَكَيْفَ يَكُونُ فِي هَذَا كَفَّارَةٌ فَقَالَ ابْنُ عَبَّاسٍ إِنَّ اللَّهَ تَعَالَى قَالَ وَ ‏{‏الَّذِينَ يُظَاهِرُونَ مِنْكُمْ مِنْ نِسَائِهِمْ‏}‏ ثُمَّ جَعَلَ فِيهِ مِنَ الْكَفَّارَةِ مَا قَدْ رَأَيْتَ ‏.‏

Yahya related to me from Malik that Yahya ibn Said heard al-Qasim ibn Muhammad say, "A woman came to Abdullah ibn Abbas and said, 'I have vowed to sacrifice my son.' Ibn Abbas said, 'Do not sacrifice your son. Do kaffara for your oath.' An old man with Ibn Abbas said, 'What kaffara is there for this?' Ibn Abbas said, 'Allah the Exalted said, "Those of you who say, regarding their wives.'Be as my mother's back' (Sura58 ayat 2) and then He went on to oblige the kaffara for it as you have seen