Hadith 443
Abu Hurairah (May Allah be pleased with him) reported:
وعن أبي هريرة، رضي الله عنه ، أن رسول الله صلى الله عليه وسلم قال" لو يعلم المؤمن ما عند الله من العقوبة، ما طمع بجنته أحد، ولو يعلم الكافر ما عند الله من الرحمة، ما قنط من جنته أحد” ((رواه مسلم)).
Messenger of Allah (ﷺ) said, "If a believer had full knowledge of the chastisement of Allah, none would covet His Jannah; and were an infidel to know the Mercy Allah has, none would despair of His Jannah". [Muslim].
In plain words
If believers fully grasped Allah's punishment, none would presume on Paradise, and if disbelievers fully grasped His mercy, none would lose hope. The heart is meant to hold fear and hope together.
Reference : Riyad as-Salihin 443 In-book reference : Introduction, Hadith 443