Assuming that their service involved tzedaka you can't get better than Isaiah 58:7-8.
ז הלוא פרוס לרעב לחמך, ועניים מרודים תביא בית: כי-תראה ערום וכיסיתו, ומבשרך לא תתעלם. ח אז ייבקע כשחר אורך, וארוכתך מהרה תצמח; והלך לפניך צדקך, כבוד יהוה יאספך.
Will you not deal out your own bread to the hungry and bring [even] the unpleasant poor into your home: when you see the naked you cover them nor ignore your own family. (8) Then your light shall break like dawn, your success will be immediate, your reputation will preceed you and God Himself will be your reward. (My own slightly free translation.)
This passage speaks movingly about the virtue of charity and how someone who practices it properly is rewarded. Try Bava Basra (bavli) around 9b-10a for many good derashot.