Is praying for someone who has rendered a service obligatory, and must it be said aloud or in secret?

Rewarding good deeds is commendable. If one is unable to do so, it is recommended for them to supplicate for the one who performed the good deed. There is no known statement indicating the obligation of supplicating or making the supplication audible to the doer of the good deed. It is recommended to supplicate for the host of food and drink, asking for blessings. If they are unable to reciprocate, then supplicating for them is considered a recompense for their generosity.