Sunan Abi Dawud, Hadith 2455 ⓘ
Abu Hurairah reported Messenger of Allah as saying: When one of you is invited (to a meal), and he is fasting, he should say that he is fasting.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا سُفْيَانُ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا دُعِيَ أَحَدُكُمْ إِلَى طَعَامٍ وَهُوَ صَائِمٌ فَلْيَقُلْ إِنِّي صَائِمٌ " .
also referenced under: Hadith 2461 (DarusSalam)