Musnad Ahmad 31:192

Yahya narrated to us, from al-Muthanna, Qatadah narrated to us, from Abu Isa al-Aswari, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "Visit the sick, and walk with the funeral processions, for they remind you of the Hereafter."