Question:

It is well known that the Final Tasha-hud is a pillar from the pillars of prayer; and in one of the prayers I did not complete except a little bit of the Tasha-hud, so should I repeat my prayer?

Answer:

It is upon you to complete the Tasha-hud, even if you missed some (of it whilst praying with) your Imaam (i.e. the one leading you in Salaah); because according to the most correct saying (or view) of the scholars the Final Tasha-hud is a pillar (of the prayer), and there is the Salutation upon the Prophet (sallal-laahu-alayhi-wasallam) in it. Therefore, it is obligatory to complete it even after the Imaam’s Salaam (i.e. after he finishes the prayer). And from it (i.e. the Tasha-hud) is seeking Allaah’s protection from the Hell fire, the punishment of the grave and from the trials of life and death, and from the trial of the Dajjaaj; because the Prophet (sallal-laahu-alayhi-wasallam) commanded with seeking (Allaah’s) protection from these four (things) in the Final Tasha-hud, and that is because some of the people of knowledge consider that to be an obligation.

والله ولي التوفيق

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Source: http://www.binbaz.org.sa/mat/979