Say “I believe in Allah”, then be steadfast

On the authority of Abu Amr — and it is also said Abu Amra — Sufyan ibn Abdillah (may Allah be pleased with him), who said: “O Messenger of Allah, tell me something about Islam about which I need ask no one after you.” He (may Allah bless him and grant him peace) said: "Say: “I believe in Allah” — and then be steadfast."

Related by Muslim.

[Muslim 38]