The Prophet said: "When one of you is invited to eat then let him respond, if he is fasting then let him pray." Meaning: supplicate.  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا دُعِيَ أَحَدُكُمْ إِلَى طَعَامٍ فَلْيُجِبْ فَإِنْ كَانَ صَائِمًا فَلْيُصَلِّ يَعْنِي الدُّعَاءَ  

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

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