Thus, if the follower omits the remembrance that belongs to the rising posture (iʿtidāl), he ends up declaring instead the one that belongs to full standing (istiwāʾ). This is plainly observable among the vast majority of worshipers: scarcely do they hear the imam say, “Samʿa Allāhu liman ḥamidah,” before they have already rushed to say, “Rabbana wa laka al-ḥamd.” By doing so they are in open conflict with the ḥadīth. Should one of them attempt to avoid that breach, he falls into another—leaving the iʿtidāl entirely devoid of its prescribed remembrance, with no evidence to justify it.

Al-Nawawī (al-Majmūʿ 3/420) said:
“Prayer is founded on the principle that no part of it be empty of remembrance. If both phrases are not uttered—one during the rise and the other after standing upright—then one of the two positions will be left without any remembrance.”

Indeed, I go further and contend that pronouncing the tasmiʿ—“Samʿa Allāhu liman ḥamidah”—during iʿtidāl is obligatory for every person who prays. This is established by the ḥadīth of the man who performed his prayer poorly, in which the Prophet ﷺ said:
“None of you completes the Prayer until he perfects his ablution as Allah has commanded… then pronounces the takbīr… then bows until his joints are at rest, and then says: ‘Samʿa Allāhu liman ḥamidah,’ after which he stands upright until his spine is straight…”
This was related by Abū Dāwūd and al-Nasāʾī—the wording belongs to al-Nasāʾī—and others, with a sound chain. It is authenticated in Sahih Abi Dawud, 804.

After all this, can anyone still claim that the tasmiʿ is not obligatory for every worshiper?