It is narrated from Sayyiduna Salman al-Farisi (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The Muslim prays while his sins are raised upon his head; whenever he prostrates, they fall away, until when he finishes his prayer, his sins have fallen away."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6100