It states (Tehillim 41:10-11) אשר שלומי איש וגם והקמני חנני 'ה ואתה עקב עלי הגדיל לחמי אוכל בו בטחתי להם ואשלמה, "Even my ally, in whom I trusted, who eats my bread, developed an ambush for me. But You, Hashem, be gracious to me and raise me up, so that I may repay them." In what way did Dovid want to repay those who went against him? The simple meaning is that I should repay them with revenge and give them what they deserve. Indeed, that is how Metzudas Dovid explains it. However, Reb Sadyah Gaon says that Dovid requested להם ואשלמה, that I should repay them with kindness.
The following pasuk states כי ידעתי בזאת בי חפצת, "With this I shall know that You desired me..." Reb Sadya Gaon explains that if I am kind to them, despite all the bad they did for me, I am certain that you will desire and love me.
Reb Sadya Gaon concludes, לא 'ה דבר והוא תטור ולא תקום, this is the meaning of the obligation that we mustn’t take revenge or hold a grudge.
