Abdullah bin Ma'qil (may Allah have mercy on him) said: I was with my father (Ma'qil, may Allah be pleased with him) beside Sayyiduna Ibn Mas'ud (may Allah be pleased with him), and he said to Sayyiduna Ibn Mas'ud (may Allah be pleased with him): Did you hear it from the Prophet (peace and blessings be upon him)? He said: Yes, I heard the Prophet (peace and blessings be upon him); he (peace and blessings be upon him) said: "Remorse is repentance."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20559