Is the tasbih in ruku' and sujud obligatory or recommended?

The questioner : Shaykh, in the book al-Nihayah, regarding the mention of the word "salat al-sabihah," it says: "And the voluntary prayer was specifically designated for the sabihah, even though the obligatory prayer shares with it in the tasbih, because the tasbihs in the obligatory prayers are among the voluntary acts." My question is about his statement that the tasbihs in the obligatory prayers are voluntary acts. Do you see this statement?

The Shaykh : Yes, the tasbih is obligatory and it is not a pillar.

Another questioner : The tasbih is obligatory and it is not a pillar.

The Shaykh : I hear you.

Another questioner : The Shaykh : He says the tasbih is obligatory and not a pillar, and because he says they are voluntary acts, you are saying it is not a pillar and that it is not a voluntary act nor a regular voluntary prayer.

The Shaykh : We say it is obligatory, but it is not a pillar and it is not recommended.