﴿ فَإِن كُنتَ فِى شَكٍّۢ مِّمَّآ أَنزَلْنَآ إِلَيْكَ فَسْـَٔلِ ٱلَّذِينَ يَقْرَءُونَ ٱلْكِتَٰبَ مِن قَبْلِكَ ۚ لَقَدْ جَآءَكَ ٱلْحَقُّ مِن رَّبِّكَ فَلَا تَكُونَنَّ مِنَ ٱلْمُمْتَرِينَ ﴾

“AND SO, [O man,] if thou art in doubt about [the truth of] what We have [now] bestowed upon thee from on high, ask those who read the divine writ [revealed] before thy time: [and thou wilt find that,] surely, the truth has now come unto thee from thy Sustainer. Be not, then, among the doubters -”

(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.