Muwatta Malik, Book 47, Hadith 7 (USC-MSA)

Yahya related to me from Malik that Yahya ibn Said said that he heard Said ibn al-Musayyab say, "Shall I tell you what is better than much prayer and sadaqa?" They said, "Yes." He said, "Mending discord. And beware of hatred - it strips you (of your deen)."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّهُ قَالَ سَمِعْتُ سَعِيدَ بْنَ الْمُسَيَّبِ، يَقُولُ أَلاَ أُخْبِرُكُمْ بِخَيْرٍ، مِنْ كَثِيرٍ مِنَ الصَّلاَةِ وَالصَّدَقَةِ قَالُوا بَلَى ‏.‏ قَالَ إِصْلاَحُ ذَاتِ الْبَيْنِ وَإِيَّاكُمْ وَالْبِغْضَةَ فَإِنَّهَا هِيَ الْحَالِقَةُ ‏.‏

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also referenced under: Book 47, Hadith 1642 (In-Book)