Sayyiduna Sufyan bin Abdullah Thaqafi (may Allah be pleased with him) narrates: I said, "O Messenger of Allah (peace be upon him)! Tell me about something which I may hold firmly." The Prophet (peace be upon him) said, "Say: My Lord is Allah, then remain steadfast." I said, "O Messenger of Allah (peace be upon him)! What do you fear most for me?" The Prophet (peace be upon him) held his tongue and then said, "This."
The narrator named Ma'iz is Ma'iz bin Abdur Rahman; this has been stated by Zubaidi, and this narrator is reliable.