Sunan Ibn Majah, Volume 5, Book 34, Hadith 3860 (DarusSalam) ⓘ
It was narrated from Abu Hurairah that : the Messenger of Allah (saas) said: "Allah has ninety-nine Names, one hundred less one. Whoever counts them will enter Paradise."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، عَنْ مُحَمَّدِ بْنِ عَمْرٍو، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " إِنَّ لِلَّهِ تِسْعَةً وَتِسْعِينَ اسْمًا مِائَةً إِلاَّ وَاحِدًا مَنْ أَحْصَاهَا دَخَلَ الْجَنَّةَ " .
also referenced under: Hadith 3860 (Reference) & Book 34, Hadith 34 (In-Book)