Musnad Ahmad 27:1253

Aswad ibn Amir Shadhan narrated to us, Abu Bakr ibn Ayyash informed us, from Layth, from Mujahid, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever asks you by Allah, then give him. Whoever invites you, then answer him. And whoever gives you a gift, then recompense him. If you do not find what to recompense him with, then supplicate for him."