Musnad Ahmad 27:916

'Affan narrated to us, Abu 'Awanah narrated to us, Sulayman al-A'mash narrated to us, from Mujahid, from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him) who said: "Whoever seeks refuge by Allah, then grant him refuge. Whoever asks of you by Allah, then give him. Whoever calls you, then answer him. Whoever does a kindness to you, then recompense him. If you do not find what to recompense him with, then supplicate for him until you know that you have recompensed him."