Jami At Tirmidhi 781
The Book on Fasting
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِذَا دُعِيَ أَحَدُكُمْ وَهُوَ صَائِمٌ فَلْيَقُلْ إِنِّي صَائِمٌ " . قَالَ أَبُو عِيسَى وَكِلاَ الْحَدِيثَيْنِ فِي هَذَا الْبَابِ عَنْ أَبِي هُرَيْرَةَ حَسَنٌ صَحِيحٌ .
Abu Hurairah narrated that :The Prophet said: "When one of you is invited (to a meal) and he is fasting, then let him said: 'Indeed I am fasting
Grading: Ahmad Muhammad Shakir: Sahih · Al-Albani: Sahih · Bashar Awad Maarouf: Hasan Sahih · Zubair Ali Zai: Sahih Muslim
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.