Musnad Ahmad 50:3

Abu Kamil narrated to us, Ibrahim—meaning Ibn Sa’d—narrated to us, Ibn Shihab narrated to us, and Yazid ibn Harun said: Ibrahim informed us, he said: Ibn Shihab narrated to me, from Muhammad ibn Abd al-Rahman ibn Ma’iz al-Amiri, from Sufyan ibn Abd Allah al-Thaqafi, who said: I said: "O Messenger of Allah, tell me of a matter I may hold fast to." He said: "Say: 'My Lord is Allah,' then remain steadfast." I said: "O Messenger of Allah, what is the greatest thing you fear for me?" He said: The Messenger of Allah, peace and blessings of Allah be upon him, took hold of his own tongue, then said: "This." Yazid said in his narration: "By the tip of his own tongue."