Tafsir of Ash-Shu`ara' 26:194

على قلبك

Upon your heart, `O Muhammad, free from any contamination, with nothing added or taken away.'

لتكون من المنذرين

that you may be of the warners, means, `so that you may warn people with it of the punishment of Allah for those who go against it and disbelieve in it, and so that you may give glad tidings with it to the believers who follow it.'

بلسان عربي مبين