Staying steadfast on Tawhid

I said: O Messenger of Allah, tell me about Islam a thing something after which I would not need to ask anyone else after you – and in the hadith of Abū Usāmah the ˹words˺ are: other than you – He said: "Say: I believe in Allah, and then remain steadfast upon it."  

قُلْتُ: يَا رَسُولَ اللَّهِ قُلْ لِي فِي الإِسْلاَمِ قَوْلاً لاَ أَسْأَلُ عَنْهُ أَحَدًا بَعْدَكَ؟ – وَفِي حَدِيثِ أَبِي أُسَامَةَ: غَيْرَكَ – قَالَ: «قُلْ آمَنْتُ بِاللَّهِ فَاسْتَقِمْ۔»