Q. If I am following the Imaam or reading Salaah alone and I did not recite the Tashhuud because of no concentration, is my Salaah valid?
A. The recital of Tashahhud in Salaah is necessary (Waajib). However, if a follower did not recite the Tashahhud in Salaah whilst performing Salaah behind the Imaam, the Salaah will remain valid as the Imaams recital accounts for the followers in Salaah.
On the other hand, if one did not recite the Tashahhud due to lack of concentration or forgetfulness whilst performing Salaah individually (Munfarid), the performance of Sajdah Sahw will be necessary to compensate for it.
In this case, if Sadjah Sahw is not performed, it will be necessary to repeat the entire Salaah. (Shaami 2/79-Ahsanul Fataawa 3/291)
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa
This answer was collected from TheMufti.com, which is a fatwa portal managed by Mufti Ismaeel Bassa from South Africa.