Tafsir of Az-Zukhruf 43:21

The Idolators have no Proof Allah condemns the idolators for worshipping others instead of Allah with no evidence or proof for doing so

أم آتيناهم كتابا من قبله

Or have We given them any Book before this? means, before their idolatry.

فهم به مستمسكون

to which they are holding fast, means, with regard to what they are doing. This is not the case. This is like the Ayah:

أم أنزلنا عليهم سلطنا فهو يتكلم بما كانوا به يشركون

Or have We revealed to them an authority (a Scripture), which speaks of that which they have been associating with Him? (30:35) meaning, that did not happen. Then Allah says:

بل قالوا إنا وجدنا آباءنا على أمة وإنا على آثارهم مهتدون