If the believer were to know what is with Allah of punishment, no one would hope to enter Paradise, and if the non-believer were to know what is with Allah of mercy, no one would despair of entering Paradise
Authentic hadithعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: «لو يعلمُ المؤمنُ ما عند الله من العقوبة، ما طَمِع بِجَنَّتِهِ أحدٌ، ولو يَعلمُ الكافرُ ما عند الله من الرَّحمة، ما قَنَطَ من جَنَّتِهِ أحدٌ».
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If the believer were to know what is with Allah of punishment, no one would hope to enter Paradise, and if the non-believer were to know what is with Allah of mercy, no one would despair of entering Paradise."
Explanation
Hadīth explanation: The believer who is promised Paradise (in the Qur'an and Sunnah), if he had true knowledge of the punishment and torture that Allah has prepared for the people of disbelief and falsehood, he would not have any hope of entering Paradise; rather, his only hope would be to be saved from Hellfire, given what it contains of severe torture that human bodies would cannot bear. Contrary to that, if the disbeliever had true knowledge of the mercy of Allah, the Most Merciful and Most Generous, he would not despair of Allah's mercy; rather he would hope to enter Paradise because of what he knows about Allah's vast and extensive mercy and forgiveness.
Source · Narrated by Muslim
From the Hadith Encyclopedia (hadeethenc.com).