It (your turning towards the Sacred Mosque, O Messenger,) is the truth from your Lord, and only that which is from your Lord is the truth; be not, then, among the doubters
The Truth (the absolute and unequivocal reality) is from your Rabb (the output of the Name composition constituting one’s brain) so never be among the doubters (of this truth).
Sayyed Abbas Sadr-Ameli
The Truth is from your Lord, therefore be not one of the doubters
Al-muntakhab fi tafsir al-Qur'an al-Karim
But the truth has proceeded to you, O Muhammad, from Allah, your Creator, therefore do not permit their false assertions to cause you doubt and anxiety