Musnad Ahmad 50:2

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Ya'la ibn 'Ata, from 'Abdullah ibn Sufyan, from his father, who said: "O Messenger of Allah, tell me a matter in Islam about which I will not ask anyone after you." He said: "Say: 'I believe in Allah,' then remain steadfast." He said: "O Messenger of Allah, and what thing should I fear?" He said: "Then he pointed with his hand to his tongue."