Al-Kāfi - Volume 1 > The degree of the Great Names of Allah that are given to the Imams (a.s.)
Hadith #3

3ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ الاشْعَرِيُّ عَنْ مُعَلَّى بْنِ مُحَمَّدٍ عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ عَبْدِ الله عَنْ عَلِيِّ بْنِ مُحَمَّدٍ النَّوْفَلِيِّ عَنْ أَبِي الْحَسَنِ صَاحِبِ الْعَسْكَرِ (عَلَيْهِ السَّلام) قَالَ سَمِعْتُهُ يَقُولُ اسْمُ الله الاعْظَمُ ثَلاثَةٌ وَسَبْعُونَ حَرْفاً كَانَ عِنْدَ آصَفَ حَرْفٌ فَتَكَلَّمَ بِهِ فَانْخَرَقَتْ لَهُ الارْضُ فِيمَا بَيْنَهُ وَبَيْنَ سَبَإٍ فَتَنَاوَلَ عَرْشَ بِلْقِيسَ حَتَّى صَيَّرَهُ إِلَى سُلَيْمَانَ ثُمَّ انْبَسَطَتِ الارْضُ فِي أَقَلَّ مِنْ طَرْفَةِ عَيْنٍ وَعِنْدَنَا مِنْهُ اثْنَانِ وَسَبْعُونَ حَرْفاً وَحَرْفٌ عِنْدَ الله مُسْتَأْثِرٌ بِهِ فِي عِلْمِ الْغَيْبِ.

3. al-Husayn ibn Muhammad al-Ash‘ari from Mu‘alla ibn Muhammad from Ahmad ibn Muhammad ibn ‘Abdallah from Ali ibn Muhammad al-Nawfali who has said that he heard abu al-Hassan (a.s.) Sahib al-‘Askar say the following. “The greatest name of Allah has seventy three letters. There was only one letter with Asif. He spoke with it and the land between him and the throne of the Queen of Sheba (Bilqis) sunk down as such that he took her throne and placed it before Solomon. The land then came to normal state within less than a blinking of an eye. There are seventy-two of those letters with us. One letter is with Allah which, He has kept it exclusively in the knowledge of the unseen.”