Say, “Call upon Allah, or call upon the Most Gracious. Whichever [name] you call – to Him belong the most beautiful names.” And do not recite [too] loudly in your prayer or [too] quietly but seek a way in between. Al-Isra 110
Abu Huraira reported Allah’s Messenger (ﷺ) as saying: There are ninety-nine names of Allah; he who commits them to memory would get into Paradise. Verily, Allah is Odd (He is one, an odd number) and He loves odd number. And in the narration of Ibn ‘Umar (the words are):” He who enumerated them.”
حَدَّثَنَا عَمْرٌو النَّاقِدُ، وَزُهَيْرُ بْنُ حَرْبٍ، وَابْنُ أَبِي عُمَرَ، جَمِيعًا عَنْ سُفْيَانَ، – وَاللَّفْظُ لِعَمْرٍو – حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ” لِلَّهِ تِسْعَةٌ وَتِسْعُونَ اسْمًا مَنْ حَفِظَهَا دَخَلَ الْجَنَّةَ وَإِنَّ اللَّهَ وِتْرٌ يُحِبُّ الْوِتْرَ ” . وَفِي رِوَايَةِ ابْنِ أَبِي عُمَرَ ” مَنْ أَحْصَاهَا ” .
Reference : Sahih Muslim 2677a
In-book reference : Book 48, Hadith 5
USC-MSA web (English) reference : Book 35, Hadith 6475
Narrated Abu Huraira: Allah has ninety-nine Names – one hundred minus one – and whoever memorizes them will enter Paradise; and Allah is witr (one) and loves witr (odd numbers).
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، قَالَ حَفِظْنَاهُ مِنْ أَبِي الزِّنَادِ عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، رِوَايَةً قَالَ “ لِلَّهِ تِسْعَةٌ وَتِسْعُونَ اسْمًا، مِائَةٌ إِلاَّ وَاحِدًا، لاَ يَحْفَظُهَا أَحَدٌ إِلاَّ دَخَلَ الْجَنَّةَ، وَهْوَ وَتْرٌ يُحِبُّ الْوَتْرَ ”.
Reference : Sahih al-Bukhari 6410
In-book reference : Book 80, Hadith 105
USC-MSA web (English) reference : Vol. 8, Book 75, Hadith 419
Quoted from https://islamqa.info/en/answers/41003/the-names-of-allaah-are-not-limited-to-ninety-nine
Al-Nawawi (may Allah have mercy on him) stated in Sharh Saheeh Muslim that the scholars were unanimously agreed on that, and he said: The scholars are unanimously agreed that this hadeeth does not mean that Allaah has only ninety-nine names, or that He does not have any other names apart from these ninety-nine. Rather what the hadeeth means is that whoever learns these ninety-nine will enter Paradise. The point is that one may enter Paradise by learning them, not that the number is limited to these names.
And Shaykh Ibn ‘Uthaymeen was asked about that and replied: The names of Allaah are not limited to a certain number. The evidence for that is the words of the Prophet (peace and blessings of Allaah be upon him) in the saheeh hadeeth: “O Allaah, I am Your slave, son of Your slave… I ask You by every name belonging to You which You have named Yourself with, or revealed in Your Book, or You taught to any of Your creation, or You have preserved in the knowledge of the Unseen with You.”
What Allaah has preserved in the knowledge of the unseen with Him cannot be known and what is not known is unlimited.
With regard to the words of the Prophet (peace and blessings of Allaah be upon him), “Allaah has ninety-nine names, one hundred less one. Whoever learns them will enter Paradise,” this does not mean that He does not have any names apart from these, rather it means that whoever learns these ninety-nine of His names will enter Paradise. This is like when the Arabs say: “I have one hundred horses which I have prepared for jihad for the sake of Allaah,” which does not mean that the speaker has only these hundred horses, rather these hundred are prepared for this purpose.
Majmoo’ Fataawa Ibn ‘Uthaymeen, 1/122.