one might still try to read it as referring to bowing (rukuʿ), for bowing is not performed on the chins.

His words “upon the chins” (lil-adhqān) mean precisely that—upon the chins—just as He says, “He laid him down upon his forehead” (37:103), i.e., upon the forehead. The expression “upon the chins” points to a full prostration: they placed their noses along with their foreheads on the ground until even their chins touched it. They were therefore unlike someone who touches the ground only with the forehead; and a person who prostrates on his nose may not bring his chin to the ground unless he lowers himself still further.

As for the argument of those who do not deem it obligatory—citing the fact that the Prophet, peace and blessings be upon him, did not prostrate when Zayd recited Sūrah al-Najm to him, and ʿUmar’s statement: when he was on the pulpit reciting Sūrah al-Naḥl and reached the verse of prostration, he descended and prostrated, and the people prostrated with him; then on the following Friday he recited it again and, upon reaching the verse of prostration, said, “O people, we pass by verses of prostration: whoever prostrates has done what is right, and whoever does not prostrate incurs no sin.” In another wording: “When the second Friday came they craned their necks….”