Musnad Ahmad 50:1

Waki’ and Abu Mu’awiyah narrated to us, they said: Hisham ibn ‘Urwah narrated to us, from his father, from Sufyan ibn ‘Abdullah al-Thaqafi, who said: I said: "O Messenger of Allah, tell me a statement about Islam that I will not ask anyone about after you" — Abu Mu’awiyah said: "after you" — He said: "Say: I believe in Allah, then remain steadfast."