It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (ﷺ) said: "When any of you is fasting on any day, he should neither use foul language nor act ignorantly. If someone abuses him, he should say, 'I am fasting, I am fasting.'" Abu Al-Zinad (may Allah have mercy on him) added: If any of your servants is invited to food and he is fasting, he should say, "I am fasting."
Hadith Referenceالسنن المأثورة / باب ما جاء في الأذان / 289
Hadith Takhrijصحیح مسلم، الصيام، باب حفظ اللسان للصائم، رقم : 1151، سنن ابی داؤد، الصوم، باب الغيبة للصائم، رقم : 2363