Muwatta Malik, Book 18, Hadith 57 (USC-MSA)

Yahya related to me from Malik from Abu'z Zinad from al-A'raj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "Fasting is a protection for you, so when you are fasting, do not behave obscenely or foolishly, and if any one argues with you or abuses you, say, 'I am fasting. I am fasting.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الصِّيَامُ جُنَّةٌ فَإِذَا كَانَ أَحَدُكُمْ صَائِمًا فَلاَ يَرْفُثْ وَلاَ يَجْهَلْ فَإِنِ امْرُؤٌ قَاتَلَهُ أَوْ شَاتَمَهُ فَلْيَقُلْ إِنِّي صَائِمٌ إِنِّي صَائِمٌ ‏" ‏ ‏.‏

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also referenced under: Book 18, Hadith 690 (In-Book)