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