Musnad Ahmad 32:839

Al-Hasan ibn Musa narrated to us, he said: I heard Hilal ibn Abi Dawud al-Habati, Abu Hisham, say: My brother, Harun ibn Abi Dawud, narrated to me, he said: I came to Anas ibn Malik and said: "O Abu Hamzah, the place is far, and we like to visit you." He raised his head and said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Any man who visits a sick person is merely wading into mercy. When he sits with the sick person, mercy engulfs him." He said: I said: "O Messenger of Allah, this is for the healthy person who visits the sick, but what is for the sick person?" He said: "His sins are cast off from him."