Dua for one who offers food or drink
اللّهُـمَّ أَطْعِمْ مَنْ أَطْعَمَني، وَاسْقِ مَنْ...
اللّهُـمَّ أَطْعِمْ مَنْ أَطْعَمَني، وَاسْقِ مَنْ سَقاني
Allahumma atAAim man atAAamanee wasqi man saqanee
Translation
O Allah, feed him who fed me, and provide with drink him who provided me with drink.
When to recite
Said to one who offers you food or drink.
Source / Reference
Muslim 3/126