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