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It was narrated that Maimunah the freed (female) slave of the Messenger of Allah (ﷺ), said: “The Prophet (ﷺ) was asked about a man who kissed his wife when they were both fasting. He said: ‘They have broken their fast.’”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا الْفَضْلُ بْنُ دُكَيْنٍ، عَنْ إِسْرَائِيلَ، عَنْ زَيْدِ بْنِ جُبَيْرٍ، عَنْ أَبِي يَزِيدَ الضِّنِّيِّ، عَنْ مَيْمُونَةَ، مَوْلاَةِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَتْ: سُئِلَ النَّبِيُّ ـ صلى الله عليه وسلم ـ عَنْ رَجُلٍ قَبَّلَ امْرَأَتَهُ وَهُمَا صَائِمَانِ قَالَ: " قَدْ أَفْطَرَا " .
sunnah.com