Sufyan ibn Abdullah reported: I said, “O Messenger of Allah, tell me about a word in Islam after which I will not need to ask anyone besides you.” The Prophet, peace and blessings be upon him, said, “Say: I have faith in Allah. Then remain steadfast to it.”

Source: Sahih 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 وَفِي حَدِيثِ أَبِي أُسَامَةَ غَيْرَكَ قَالَ قُلْ آمَنْتُ بِاللَّهِ فَاسْتَقِمْ

38 صحيح مسلم كِتَاب الْإِيمَانِ باب جَامِعِ أَوْصَافِ الْإِسْلَامِ