Musnad Ahmad 27:301

Muhammad ibn Ubayd narrated to us, from Yusuf ibn Suhayb, from Zayd al-Ammi, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever wishes for his supplication to be answered and his distress to be relieved, let him relieve one who is in hardship."