Hadith #21 (The Forty Hadith of Imam Nawawi)
Book: Say: I believe in Allah, then be steadfast
Narrator(s): Abu Amr Sufyan ibn Abdullah ath-Thaqafee
Grade: Sahih

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.”

Source: https://parallelquran.com/hadith/forty-nawawi-21/