It was narrated from Abu Hurairah that the Messenger of Allah ﷺ said:
“When anyone of you is fasting, let him not utter evil or ignorant speech. If anyone speaks to him in an ignorant manner, let him say: ‘I am fasting.’”

حَدَّثَنَا مُحَمَّدُ بْنُ الصَّبَّاحِ، أَنْبَأَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ ﷺ ـ " إِذَا كَانَ يَوْمُ صَوْمِ أَحَدِكُمْ فَلاَ يَرْفُثْ وَلاَ يَجْهَلْ فَإِنْ جَهِلَ عَلَيْهِ أَحَدٌ فَلْيَقُلْ إِنِّي امْرُؤٌ صَائِمٌ " .