Sufyan ibn Abdullah reported: I said, “O Messenger of Allah, tell me of a matter I can depend upon?” The Messenger of Allah, peace and blessings be upon him, said, “Say: My Lord is Allah. Then, remain upright.” I said, “O Messenger of Allah, what do you fear most for me?” The Prophet pointed to his tongue and then he said, “This.”
Sunan al-Tirmidhī 2410