It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The expiation of sins is remorse."
Hadith Referenceالمعجم الأوسط / باب الميم / 5072
Hadith Takhrij«أخرجه أحمد فى «مسنده» برقم: 2666، والطبراني فى «الكبير» برقم: 12795»