Sahih Al Bukhari – Book of Oneness, Uniqueness Of Allah (Tawheed)
Volume 009, Book 093, Hadith Number 489.
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Narrated By Abu Huraira RA: Allah's Apostle PBUH 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.
Junaid Tahir