Sahih al-Bukhari, Book 97, Hadith 21 (In-Book) ⓘ
Narrated Abu Huraira: Allah's Messenger said, "Allah has ninety-nine Names, one-hundred less one; and he who memorized them all by heart will enter Paradise." To count something means to know it by heart.
حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، حَدَّثَنَا أَبُو الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِنَّ لِلَّهِ تِسْعَةً وَتِسْعِينَ اسْمًا مِائَةً إِلاَّ وَاحِدًا، مَنْ أَحْصَاهَا دَخَلَ الْجَنَّةَ " . {أَحْصَيْنَاهُ} حَفِظْنَاهُ.
also referenced under: Hadith 7392 (DarusSalam) & Volume 9, Book 93, Hadith 489 (USC-MSA)