When must the prostrations of forgetfulness (Sujud as-Sahw) be performed in cases of omissions or additions?

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The prostration of forgetfulness must be performed before the final Taslim (greeting) in all the aforementioned instances. This is supported by the action of Mu'awiyah and the established practice of the Prophet, as narrated by 'Abdullah ibn Malik ibn Buhaynah: "The Prophet led them in the Dhuhr prayer and stood up in the first two rak'ahs without sitting for the first Tashahhud. The people followed him. When he finished the prayer and the people awaited his Taslim, he said Takbir while seated, and performed two prostrations before greeting them (Taslim)." This hadith is agreed upon (Muttafaq 'alayh).