Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “Allah has ninety nine names and whoever preserves them will enter Paradise.”

Source: Sahih Muslim 2677

Grade: Sahih (authentic) according to Muslim

عَنْ أَبِي هُرَيْرَةَ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ لِلَّهِ تِسْعَةٌ وَتِسْعُونَ اسْمًا مَنْ حَفِظَهَا دَخَلَ الْجَنَّةَ 

2677 صحيح مسلم كِتَاب الذِّكْرِ وَالدُّعَاءِ وَالتَّوْبَةِ وَالِاسْتِغْفَارِ لله تسعة وتسعون اسما من حفظها دخل الجنة وإن الله وتر يحب الوتر

What does 'preserve them' mean?
As-Salaamu Alaykum, it means to memorize them and also to act upon them, so for example knowing Allah is the Most Just means we should not oppress anyone. Allah knows best