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