((إِذَا دُعِيَ أَحَدُكُمْ فَلْيُجِبْ، فَإِنْ كَانَ صَائِماً فَلْيُصَلِّ، وَإِنْ كَانَ مُفْطِراً فَلْيَطْعَمْ)) ، وَمَعْنَى فَلْيُصَلِّ أَيْ فَلْيَدْعُ.
Translation
(If you are not invited (to a meal) then answer. If you happen to be fasting, then supplicate (for those present) and if you are not fasting, then eat.)
Source
Hisnul Muslim (Fortress of the Muslim), No. 185 — chapter “Invocation for someone who offers you food when you are fasting, which you decline”. hisnmuslim.com
Related Duas
Invocations for breaking the fast
Supplication 1 of 2
((ذَهَبَ الظَّمَأُ وَابْتَلَّتِ العُرُوقُ، وَثَبَتَ الْأَجْرُ إِنْ شَاءَ اللَّهُ)).
(The thirst has gone and the veins are quenched, and reward is confirmed, if Allah wills.)
Hisnul Muslim · No. 176
((أَفْطَرَ عِنْدَكُمُ الصَّائِمُونَ، وَأَكَلَ طَعَامَكُمُ الْأَبْرَارُ، وَصَلَّتْ عَلَيْكُمُ الْمَلاَئِكَةُ)).
(May the fasting break their fast in your home, and may the dutiful and pious eat your food, and may the angles send prayers upon you.)
Hisnul Muslim · No. 184
((إِنِّي صَائِمٌ، إِنِّي صَائِمٌ)).
(I am fasting, I am fasting.)
Hisnul Muslim · No. 186