It wasnarrated from Abu Hurairah that the Prophet (ﷺ) said:
"Ifanyoneof you is invited to eat when he is fasting, let him say: 'I amfasting.'" (TR)
English reference (TR): Vol. 1, Book 7, Hadith 1750 (TR)
Arabic reference (TR): Book 7, Hadith 1822 (TR)