Musnad Ahmad 50:4

'Ali ibn Ishaq narrated to us, he said: 'Abdullah—meaning Ibn al-Mubarak—informed us, he said: Ma'mar informed us, from al-Zuhri, from 'Abd al-Rahman ibn Ma'iz, from Sufyan ibn 'Abdullah al-Thaqafi, who said: I said: "O Messenger of Allah, tell me a matter that I may hold fast to." He said: "Say: 'My Lord is Allah,' then remain steadfast." I said: "O Messenger of Allah, what is the thing you fear most for me?" He said: Then he took hold of his own tongue, and said: "This."