It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever asks you by Allah, give him; whoever seeks refuge in Allah, grant him refuge; whoever invites you, accept his invitation; and whoever does you a kindness, repay him. If you do not find anything with which to repay him, then pray for him until he sees that you have repaid him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13408