Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
suyuti:8329a –
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السيوطي:٨٣٢٩a –

(إِنْ سُبَّ أَحَدُكُمْ وَهُوَ صَائِمٌ فَليَقُلْ إِنِّى صَائِمٌ) ، حب عن أَبى هريرة.  

See similar narrations below:

Collected by Tirmidhī, Nasāʾī's Kubrá
tirmidhi:781 – Naṣr b. ʿAlī > Sufyān b. ʿUyaynah > Abū al-Zinād > al-Aʿraj > Abū Hurayrah

The Prophet said: "When one of you is invited (to a meal) and he is fasting, then let him said: 'Indeed I am fasting.'"  

الترمذي:٧٨١ – حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ عَنْ أَبِي الزِّنَادِ عَنِ الأَعْرَجِ عَنْ أَبِي هُرَيْرَةَ

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا دُعِيَ أَحَدُكُمْ وَهُوَ صَائِمٌ فَلْيَقُلْ إِنِّي صَائِمٌ  

قَالَ أَبُو عِيسَى وَكِلاَ الْحَدِيثَيْنِ فِي هَذَا الْبَابِ عَنْ أَبِي هُرَيْرَةَ حَسَنٌ صَحِيحٌ
nasai-kubra:3247 – Muḥammad b. Maʿdān > al-Ḥasan b. Aʿyan Ḥarrānī Thiqah > Maʿqil > ʿAṭāʾ > Abū Hurayrah

[Machine] I heard Abu Hurairah say, "If one of you is fasting and someone insults or provokes him, let him say, 'Indeed, I am fasting.'"  

الكبرى للنسائي:٣٢٤٧ – أَخْبَرَنَا مُحَمَّدُ بْنُ مَعْدَانَ قَالَ حَدَّثَنَا الْحَسَنُ بْنُ أَعْيَنَ حَرَّانِيٌّ ثِقَةٌ قَالَ حَدَّثَنَا مَعْقِلٌ عَنْ عَطَاءٍ قَالَ

سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ إِذَا كَانَ أَحَدُكُمْ صَائِمًا فَسَابَّهُ أَحَدٌ فَلْيَقُلْ إِنِّي صَائِمٌ