In the name of Allah, Most Gracious, Most Merciful.
Abu Hurairah (May Allah be pleased with him) reported: The Prophet (PBUH) said, “He who observes fasting during the month of Ramadan with Faith while seeking its reward from Allah, will have his past sins forgiven.” (Al-Bukhari and Muslim).
Riyaadiss-Saliheen (The Meadows of the Righteous)
By Imam An Nawawi
Chapter 217, Page 355, No 1219