Musnad Ahmad 31:455

Waki’ and Bahz narrated to us, they said: Muthanna bin Sa’id narrated to us, from Qatadah; and Waki’ narrated to us, Hammam narrated to us, from Qatadah, from Abu ‘Isa, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Visit the sick, and follow the funeral processions, for they will remind you of the Hereafter."