Sunan At-Tirmidhi

Sufyan bin 'Abdullah Ath-Thaqafi said:I said: ""O Messenger of Allah! Inform me about a matter that I may hold fast to."" He said: 'Say: My Lord is Allah, then be steadfast.' I said: ""O Messenger of Allah! What do you fear most for me?"" So he took hold of his tongue and said: 'This