Others said: it is not obligatory.
Another opinion maintains that it becomes obligatory only when the verse of prostration is recited during the prayer; this is one report from Ahmad.

What seems clear to me is that it is obligatory. The verses that merely contain praise do not, by themselves, establish an obligation; but as for those that carry a command, a reproach, or are worded unconditionally, some have argued that they pertain only to the prayer—such as the second prostration verse in al-Hajj, and the verses in al-Furqan and Iqraʾ. This interpretation is weak, all the more so when the verse explicitly links the act to recitation, as in His statement:

“Only those who believe in Our signs are those who, when they are reminded of them, fall down in prostration and glorify the praise of their Lord, and they are not arrogant.” [al-Sajdah 15]

Here faith in the signs is denied to anyone who does not prostrate when reminded of them, and the mere act of listening is itself a reminder.

The same applies in Surah al-Inshiqaq:

“So what is the matter with them that they do not believe, and when the Quran is recited to them, they do not prostrate?” [al-Inshiqaq 20–21]

This is a censure of those who fail to prostrate when the Quran is recited to them, just as in His words:

“So what is the matter with them that they turn away from the reminder?” [al-Muddaththir 49]

“And what is wrong with you that you do not believe in Allah while the Messenger calls you to believe in your Lord?” [al-Hadid 8]

“So what is the matter with these people that they can hardly understand any statement?” [al-Nisaʾ 78]

Likewise, in Surah al-Najm He says:

“Is it at this discourse that you marvel, laughing and not weeping, while you are heedless? Then prostrate to Allah and worship!” [al-Najm 59–62]

a most emphatic command that follows the mention of the discourse which