Hadith 85
Sufyan bin 'Abdullah (May Allah be pleased with him) reported:
وعن أبي عمرو، وقيل: أبي عمرة سفيان بن عبد الله رضي الله عنه قال: قلت: يا رسول الله قل لي في الإسلام قولاً لا أسأل عنه أحداً غيرك. قال: "قل آمنت بالله: ثم استقم" ((رواه مسلم)).
I said, "O Messenger of Allah, tell me something of Islam which I will not ask anyone else about it." He (ﷺ) said, "Say, 'I believe in Allah' and then be steadfast". [Muslim].
In plain words
Asked for one defining teaching, the Prophet said: declare belief in Allah, then hold firmly to it. Faith followed by steadfastness sums up the whole religion.
Reference : Riyad as-Salihin 85 In-book reference : Introduction, Hadith 85