‏السلام عليكم ورحمة الله وبركاته It is known that the sins of a person who reverted are forgiven. Does the same rule apply to a person who believed in Allah however did not pray (disbeliever)then he started praying.I know it is up to Allah however i would like to know the fiqh regarding this matter

Ustadh AbdulAziz al-Haqqan

If we say he left Islam by doing so and then he repented and came back to Islam, then yes. Also, repentance removes what was done before it, as the Messenger of Allah ﷺ mentioned in the hadith: "التوبة تجب ما قبلها."