اللَّهُمَّ بَارِكْ لَهُمْ فِيمَا رَزَقْتَهُمْ واغْفِرْ لَهُمْ وارْحَمْهُمْ
Allahumma barik lahum feema razaqtahum waghfirlahum warhamhum
O Allah, bless for them, that which You have provided them, forgive them and have mercy upon them
Sharing your food with others may be one of the most common and simplest things to do, however, it is not to be taken lightly as there is an endless form of goodness that is hidden behind that simple gesture. Whenever you are invited or got food served by someone, make sure to express your gratefulness for the food and pray for them by asking for Allah's blessings as a form of thankfulness. As a Muslim who is encouraged to help each other, your dua would be the perfect reply for the food that the host had offered you, an exchange through prayer for Allah's blessings for them, and hopefully, accounted as their good deeds for Jannah.
The food that is offered in this dua, does not only limited to feasts or full-portioned meals. Any kind of food that is offered with sincerity and the intention of helping others is also included, even as small as offering dates to break a fast or a glass of water to ease someone's thirst. Therefore, it is best to always express your gratitude and recite this dua whenever possible.
Reciting the dua and praying for someone who had previously done a good deed - in this case, that had provided us with food - are one of the things that we should get used to. Quoted from the Prophet SAW, that even though that person is not aware of your prayers and gratitude for their actions, praying for other Muslims' well-being will hopefully return in the same effect on you - as angels will also pray for us to also get the goodness from the prayers that we have recited. Therefore, may reciting this dua and with Allah’s permission, His blessings would return to both who had done the good deed and the one who received them. Amin.