What is the ruling on one who omits the final Tashahhud, thinking it is sunnah according to Imam Malik?

The Tashahhud and the sitting for it are pillars of prayer according to some scholars, such as Umar, his son, and Al-Shafi'i. Malik and Abu Hanifa, however, believe it is not obligatory, though Abu Hanifa made the sitting for the duration of the Tashahhud obligatory. Those who argue against its obligation cite that the Prophet, peace and blessings be upon him, did not teach it to the Bedouin. As for those who affirm its obligation, they rely on the Prophet's command when he said, "Say: 'All salutations are for Allah...'," and that he consistently performed it, as well as on Ibn Mas'ud's hadith which indicates its obligation after it had not been obligatory. The hadith of the Bedouin could mean that it was before the Tashahhud was made obligatory, or that the Prophet left it out because he did not see any harm in its omission. The correct view is that the final Tashahhud is obligatory, but whoever omits it believing it to be merely recommended is not required to repeat past prayers, because they followed a recognized imam. Imposing qada' (making up prayers) in matters of ijtihad (independent reasoning) is very difficult. One should, however, strive to perform the prayer as commanded by the Prophet, peace and blessings be upon him.