Tafseer Ibn Kathir of AS SAJDAH [1]

1. Alif Lam Mim. 2. The revelation of the Book in which there is no doubt, is from the Lord of all that exists. 3. Or say they: ”He has fabricated it'' Nay, it is the truth from your Lord, so that you may warn a people to whom no warner has come before you, in order that they may be guided. We discussed the individual letters at the beginning of Surat Al-Baqarah, and there is no need to repeat it here.

(The revelation of the Book in which there is no doubt,) means, there is no doubt whatsoever that it has been revealed

(from the Lord of all that exists.) Then Allah tells us about the idolators:

(Or say they: ”He has fabricated it''): they say, he has fabricated it, i.e., he has made it up by himself.

(Nay, it is the truth from your Lord, so that you may warn a people to whom no warner has come before you, in order that they may be guided.) means, in order that they may follow the truth.