:wasalamex
This hadeeth is in sahih bukhari and muslim from the narration of Abu Hurayra RA.
There is another narration (رواية) of the same hadeeth which says : "who counts them will enter Jannah" ( من أحصاها دخل الجنة).
Scholars said it's not only about memorizing the 99 name or counting them (evoking or recalling them one by one). But the hadeeth is rather about learning these names, understanding the correct meaning of each one of them, believing in them, and working according to them : For example if you know that Allah is The Merciful, then you should try to profit from his mercy, and if you learn that Allah is The Forgiving, then you should ask him forgiveness, etc.