The procedure if the latecomer's missed portion requires a prostration of forgetfulness (Sujud as-Sahw) after the Salam?

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If the latecomer is required to perform a prostration of forgetfulness after the Salam for the part he catches up on, he performs the Sujud as-Sahw first, and then says the Takbir. This view is held by Al-Thawri, Al-Shafi'i, Ishaq, and the Ashab ar-Ra'y (Hanafi jurists). This ruling applies because the Sujud as-Sahw is legislated for the prayer, and thus the Takbir follows it, just as it follows the Tashahhud.