Ibn Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "The one who repents from sin is just like one who has never sinned at all."
This chain has weakness in it. It has also been narrated through another weak chain from Abu Sa'dah al-Ansari from the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20563