Sitting in Tashahhud by mistake after the first Raka'at

الاستفتاء

Someone is performing four Rakats of an obligatory Salah. After completing the two prostrations (Sujood) of the first Rakat, he remains sitting for approximately the length of three Tasbeehat. When it came to mind that he had only performed one Rakat, he immediately stands for the second Rakat. Is Sajda-Sahw necessary in this scenario? How long would he have to sit after the first Rakat before standing for the second Rakat in order for Sajda-Sahw to be necessary?

الجواب حامدا و مصلیا

One of the things that causes Sajda-Sahw to become necessary is delaying a Rukn (pillar/main element) of Namaz. The scholars have written that the extent of delay which makes Sajda-Sahw necessary is that in which a Rukn of Namaz (such as Ruku, Sajda etc.) can be fulfilled, or in which one can say “Subhana Rabee al-‘Azeem” three times. Hence, in the aforementioned scenario, since the one performing Namaz sat for such a delayed period in which a Rukn can be fulfilled, Sajda-Sahw will be necessary.
And Allah knows best