Ibn al-Anbārī also said:

“The first part of someone falling down to meet the ground, before he lowers his forehead, is his chin. Hence He says, ‘upon their chins.’ The meaning may also be that they fall upon their faces, with the chin standing for the face, just as a part may stand for the whole, or a species for its genus.”

I say:

Someone who falls down chin-first does not prostrate upon his chin, for the chin is not one of the parts of the body upon which one prostrates. Rather, these are seven, as the Prophet, may Allah bless him and grant him peace, said:

“I have been commanded to prostrate upon seven parts of the body: the forehead”—and he pointed with his hand to his nose—“the two hands, the two knees, and the two feet.”

Were someone to prostrate upon his chin, his forehead would be raised. Bringing both into contact with the ground is impossible or difficult, because the nose lies between them and protrudes, preventing both from touching the ground at the same time. Thus, the person prostrating descends chin-first and prostrates upon his forehead. This is the descent into prostration.

Then He says:

“And they fall down upon their chins, weeping.”

This is a descent in weeping, which may or may not be accompanied by prostration.

The first case is exemplified by His words:

“When the verses of the Most Merciful were recited to them, they fell down in prostration and weeping.”

Here there is falling down, prostration, and weeping.

The second case is exemplified by His words:

“And they fall down upon their chins, weeping.”

A person may weep out of fear of Allah, humbling himself by falling down, even without descending as far as prostration.