Mishkat al-Masabih 1 Faith كتاب الإيمان 41

وَعَنْ أَبِي أُمَامَةَ أَنَّ رَجُلًا سَأَلَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَا الْإِيمَانُ قَالَ إِذَا سَرَّتْكَ حَسَنَتُكَ وَسَاءَتْكَ سَيِّئَتُكَ فَأَنْتَ مُؤْمِنٌ قَالَ يَا رَسُولَ اللَّهِ فَمَا الْإِثْمُ قَالَ إِذَا حَاكَ فِي نَفْسِكَ شَيْءٌ فَدَعْهُ» . رَوَاهُ أَحْمد

Abu Umama reported that a man asked God’s messenger what faith was, to which he replied, “When your good deed pleases you and your evil deed grieves you, you are a believer.” He then asked what sin was and received the reply, “When you have a besetting sin, give it up.” Ahmad transmitted it.