Musnad Ahmad 28:124

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to me, that Abu Abd al-Rahman al-Hubuli narrated to him, from Abdullah ibn Amr ibn al-As, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When a man comes to visit a sick person, he should say: 'O Allah, heal Your servant; he will inflict a wound upon an enemy for You, and he will walk to prayer for You.'"