Musnad Ahmad 27:1293

Surayj narrated to us, Abu 'Awanah narrated to us, from al-A'mash, from Mujahid, from Ibn 'Umar, that the Prophet (peace and blessings of Allah be upon him) said: "Whoever asks you by Allah, then give him. Whoever seeks refuge with you by Allah, then grant him refuge. Whoever does a favor for you, then recompense him. If you do not find what to recompense him with, then pray for him until you know that you have recompensed him. And whoever seeks your protection, then protect him."