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.'" (FR)
English reference (FR): Vol. 1, Book 7, Hadith 1750 (FR)
Arabic reference (FR): Book 7, Hadith 1822 (FR)