Reference : Hisn al-Muslim 183

اللّهُـمَّ أَطْعِمْ مَن أَطْعَمَني، 
وَاسْقِ مَن سقاني
Allāhumma aṭ`im man aṭ`amanī 
wasqi man saqānī.


O Allah, feed the one who has fed me,
and give drink to the one who has given me drink.

Reference:
Muslim 3/126.

Chapter: To one who intends to give food or drink