Musnad Ahmad 33:1169

'Affan narrated to us, Mubarak narrated to us, Nasr ibn Rashid narrated to me, in the year one hundred, from the one who narrated to him, from Jabir ibn 'Abdullah al-Ansari, who said: "The Messenger of Allah (may Allah bless him and grant him peace) forbade that graves be plastered with gypsum, or that they be built upon."