(28) باب الصَّائِمِ يُدْعَى لِطَعَامٍ أَوْ يُقَاتَلُ فَلْيَقُلْ إِنِّي صَائِمٌ
Abu Huraira (Allah be pleased with him) 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." (ES)
Reference (ES): Sahih Muslim 1150 (ES)
In-book reference (ES): Book 13, Hadith 208 (ES)
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2562 (ES)