The Messenger of Allah ﷺ said, "If the believer knew the extent of the punishment Allah has prepared, no one would hope for His paradise. And if the disbeliever knew the extent of the mercy Allah has, no one would despair of His paradise."
أن رسول الله ﷺ 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.
Muslim reported it in the Book of Repentance (2755) through several routes from Ismail bin Jafar, who said, Ala (that is, Ibn Abdul-Rahman bin Yaqub) informed me, from his father, from Abu Hurairah, who narrated it.