قُلْ صَدَقَ اللّهُ
...
Say, "Allah has spoken the truth;"
meaning, O Muhammad, say that Allah has said the truth in what He conveyed and legislated in the Qur'an.
...
فَاتَّبِعُواْ مِلَّةَ إِبْرَاهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ
follow the religion of Ibrahim the Hanif, and he was not of the idolators."
Therefore, follow the religion of Ibrahim that Allah legislated in the Qur'an. Indeed, this is the truth, there is no doubt in it, and the perfect way, and no Prophet has brought a more complete, clear, plain and perfect way than he did.
Allah said in other Ayat,
قُلْ إِنَّنِى هَدَانِى رَبِّى إِلَى صِرَطٍ مُّسْتَقِيمٍ دِينًا قِيَمًا مِّلَّةَ إِبْرَاهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ
Say: "Truly, my Lord has guided me to a straight path, a right religion, the religion of Ibrahim, the Hanif, and he was not of the idolators." (6:161)
and,
ثُمَّ أَوْحَيْنَأ إِلَيْكَ أَنِ اتَّبِعْ مِلَّةَ إِبْرَهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ
Then, We have sent the revelation to you (saying): "Follow the religion of Ibrahim, the Hanif, and he was not of the idolaters. (16:123)