(And if thou) O Muhammad (art in doubt concerning that which We reveal unto thee) concerning that with which We sent Gabriel, i.e. the Qur'an, (then question those who read the Scripture) i.e. the Torah ((that was) before you) 'Abdullah Ibn Salam and his followers. The Prophet (pbuh) did not ask nor was he ever in doubt about the Qur'an. Rather, Allah was addressing with these words the people of the Prophet. (Verily the Truth from thy Lord) i.e. Gabriel with the Qur'an from your Lord, containing the events of past nations (hath come unto thee) O Muhammad. (So be not thou of the waverers) be not of the doubters.