Abu Sa'íd Al Khudrí (Ra) narrates: I heard Rasúlullãh (saw) saying: The five Salãt are an atonement for the minor sins committed between them. Then he said: A man goes to work and between his house and his place of work are five streams. When he reaches his place of work and labours there to the extent, that Allãh willed, he becomes dirty and sweaty. Then (on his way back) he passes by the streams, bathing in all, and this repeated bathing leaves no dirt or sweat on him. Salãt is just like that. Whenever, someone commits a sin, and performs Salãt, makes Du'ã, and asks forgiveness, then he is forgiven for the sins he committed preceding the Salãt.
(Bazzãr, Tabarãní, Mamma"uz-Zawãid)