Musnad Ahmad 27:1513

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Hisham—meaning Ibn Sa’d—narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (may Allah bless him and grant him peace) saw a man resting his hand [on the ground] while praying. He said: "Do not sit like this, for this is only the sitting of those who are being punished."