Does not the action of the Prophet (peace be upon him) of sometimes observing the jalsat al-istirahah (short sitting after the prostration) and sometimes omitting it, as well as the Companions not observing it, indicate that it is a Sunnah that is sometimes performed and sometimes left?

It was previously clarified that the sitting of rest (jalsat al-istirahah) is a Sunnah, and the Prophet's (peace and blessings be upon him) leaving it along with some Companions indicates that it is not obligatory. So, whoever performs it consistently is a doer of good, observing the Sunnah. Whoever leaves it incurs no sin. And whoever performs it sometimes and leaves it sometimes incurs no blame.