Assalamu 'alaikum I read in a book about specific dua after each prayer of fajr,dhuhr,asr etc.,but I havenot come across such dua anywhere,so please tell me if it is correct to say these duas after each prayer and if the Prophet(peace be upon him) said these.If we do what was not taught by the Prophet,it would be rejected,right?pls answer.May Allah reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

There is nothing reported from the Prophet with this meaning, and it is not permissible to say these supplications as they are included in the secondary innovations (i.e. the innovations where the basis of worship is permitted but there is some change with regard to its reason, type, amount, manner, time and place), but if the meaning of these supplications is good, and a praying person says them sometimes without believing that they are a Sunnah, then this is permissible because the supplication after performing obligatory prayers is, in principle, among the legislated matters.

Allaah Knows best.