Saying "May Allah accept your Salah'' to another who is sitting beside him after Taslim (salutation of peace ending the Prayer) is not Mashru` (Islamically acceptable); rather, it is Bid`ah (innovation in Islam). The Prophet (peace be upon him) and the Companions (may Allah be pleased with them) never did it. It was authentically reported that the Prophet (peace be upon him) stated:
Anyone who introduces anything into this matter of ours (Islam) that is not part of it will have it rejected.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.