صحيح مسلم
Sahih Muslim
كتاب الصيام
The Book of Fasting
باب نُدْبِ الصَّائِمِ إذَا دُعِيَ إِلَي الطَّعَامِ وَلَمْ يُرِدِ الْإِفْطَارَ ، أَوْ شُوتِمَ أَوْ قُوتِلَ أَنْ يَقُولَ : إِنِّي صَائِمٌ وَّأَنَّهُ يُنزِّهُ صَوْمَهُ عَنِ الرَّفَثِ وَالْجَهْلِ وَنَحْوِهِ
Chapter: If a fasting person is invited to eat and he does not want to break his fast, or someone insults him or argues with him, it is recommended for him to say: "I am Fasting", and he should protect his fast from obscene speech, ignorance and the like
1 hadith
Hadith 1150
Abu Hurairah (RA) reported Allah's Apostle (ﷺ) as saying: If any one of you is invited to a meal when he is fasting, he should say: "I am fasting."