Sayyiduna al-Hakam bin Umayr (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever does you a kindness, repay him; and if you find nothing with which to repay him, then supplicate for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 3176