Renew Your Iman...

It is narrated on the authority of Abu Hurairah (radiallaahu 'anhu) that the Messenger of Allaah (salallaahu 'alaihi wa sallam) said:  

Renew your Iman. It was said: O Messenger of Allaah! How do we renew our Iman (faith)? He said: Frequently say 'La illaaha il Allaah' (none has the right to be worshipped except Allaah).

It is reported on the authority of Abu Dharr (radiallaahu 'anhu) that he said: I said, 'O Messenger of Allaah, give me some advice.' He said:
 If you do an evil deed, then follow it up with a good deed; it will erase it... He said: 'I then asked the Messenger of Allaah: is 'La illaaha il Allaah' from the good deeds?' He said: 'It is the best of the deeds.'