Sufyan ibn Abdullah reported: I said, “O Messenger of Allah, tell me of a word in Islam I will not need to ask of anyone after you.” The Messenger of Allah, peace and blessings be upon him, said, “Say, ‘I have faith in Allah,’ and then remain steadfast.”
Source: Ṣaḥīḥ Muslim 38
Grade: Sahih (authentic) according to Muslim
عَنْ سُفْيَانَ بْنِ عَبْدِ اللَّهِ الثَّقَفِيِّ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ قُلْ لِي فِي الإِسْلاَمِ قَوْلاً لاَ أَسْأَلُ عَنْهُ أَحَدًا بَعْدَكَ قَالَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قُلْ آمَنْتُ بِاللَّهِ فَاسْتَقِمْ
38 صحيح مسلم كتاب الإيمان باب جامع أوصاف الإسلام