ALMOST all the authorities agree in that this surah belongs to the late Mecca period, and that it was revealed immediately after surah 23 ("The Believers"). The view advanced by some commentators that verses 16-20 were revealed at Medina is purely speculative and does not deserve serious consideration. The key-word which came to be accepted as the "title" of this surah is found in verse 15.
Alif. Lam. Mim.1