Short Answer

At the end of the meal, say:

اللّهُـمَّ أَطْعِمْ مَن أَطْعَمَني وَاسْقِ مَن سَقَانِي

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

(Source: Muslim 3:126)

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Shaykh Mustafa Umar