Musnad Ahmad 30:1206

Musa ibn Dawud narrated to us, Hammad ibn Salamah narrated to us, from Abu Sinan, from Uthman ibn Abi Sawdah, from Abu Hurairah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "When a Muslim visits his brother for the sake of Allah, the Mighty and Majestic, or visits him when he is sick, Allah, the Mighty and Majestic, says: 'You have become good, and you have taken for yourself a dwelling in Paradise.'"