Abdullah bin Ma'qil said that his father asked Sayyiduna Ibn Mas'ud (may Allah be pleased with him): Did you hear the Messenger of Allah (peace and blessings be upon him) say: 'An-nadamu tawbah' (Remorse is repentance)? He said: "Yes!"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20558