Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 450
Abu Huraira reported: Verily the Messenger of Allah (may peace be upon him) said: Five (daily) prayers and from one Friday prayer to the (next) Friday prayer, and from Ramadhan to Ramadhan are expiatious for the (sins) committed in between (their intervals) provided one shuns the major sins.