Muwatta Malik, Book 2, Hadith 98 (In-Book)

Yahya related to me from Malik that Ibn Shihab used to say, "Wudu is necessary if a man kisses his wife." Nafi said that Malik said, "That is what I like most out of what I have heard."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ كَانَ يَقُولُ مِنْ قُبْلَةِ الرَّجُلِ امْرَأَتَهُ الْوُضُوءُ ‏.‏ قَالَ ‏{‏ ابْنُ ‏}‏ نَافِعٍ قَالَ مَالِكٌ وَذَلِكَ أَحَبُّ مَا سَمِعْتُ إِلَىَّ ‏.‏

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also referenced under: Book 2, Hadith 68 (USC-MSA)