On the authority of Abu `Amr — and he is also called Abu `Amrah — Sufyan bin Abdullah ath- Thaqafee (may Allah be pleased with him) who said: I said, "O Messenger of Allah, tell me something about al-Islam which I can ask of no one but you." He (peace and blessings of Allah be upon him) said, "Say I believe in Allah — and then be steadfast." [Muslim]
Forty Hadith of an-Nawawi · no. 21