The Book of Repentance
Ṣaḥīḥ Muslim 6979
لَوْ يَعْلَمُ الْمُؤْمِنُ مَا عِنْدَ اللَّهِ مِنَ الْعُقُوبَةِ مَا طَمِعَ بِجَنَّتِهِ أَحَدٌ وَلَوْ يَعْلَمُ الْكَافِرُ مَا عِنْدَ اللَّهِ مِنَ الرَّحْمَةِ مَا قَنِطَ مِنْ جَنَّتِهِ أَحَدٌ
law yaʿlamu al-muʾminu mā ʿinda-Llāh mina al-ʿuqūbah mā ṭamiʿa bijannatihi aḥadun walaw yaʿlamu al-kāfiru mā ʿinda-Llāh mina ar-raḥmah mā qaniṭa min jannatihi aḥadun
Transliteration generated from the Arabic vowel marks
The full narration
Ala' reported on the authority of his father who reported on the authority of Abu Huraira that Allah's Messenger (ﷺ) said:If a believer were to know the punishment (in Hell) none would have the audacity to aspire for Paradise (but he would earnestly desire to be rescued from Hell), and if a non-believer were to know what is there with Allah as a mercy. none would have been disappointed in regard to Paradise
Source
Ṣaḥīḥ Muslim · 6979