Musnad Ahmad 27:1646

'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 you, then give him. Whoever calls you, then answer him. Whoever does a favor for 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."