How should a person perform Ruku’ and Sajdah whilst sitting and performing Salaah?

It is permissible to perform all Sunnah and Nafl Salaah whilst sitting on the ground and making Sajdah on the ground. If there is no valid reason for sitting on the ground the reward of the Salaah is halved.

With regards to performing Sunnah and Nafl Salaah on a chair the following rules apply:

a) If a person is sitting on a chair and performing Salaah without a valid excuse, then it remains obligatory (Fardh) to perform the Sajdah by placing the face on the ground. In this case, one will receive half the reward of a person standing and performing Salaah.

b) If a person is sitting on a chair and performing Salaah with a valid excuse which prevents making Sajdah on the ground, then one may perform the Ruku’ and Sajdah by gestures (Ishaarah) e. by bending the upper body in such a way that the gesture for Sajda is lower than the gesture for Ruku’. In this case, one will receive the full reward of performing Salaah.

In the case of the Five Fardh (obligatory) Salaahs, it will not be permissible to perform them whilst sitting down without a valid reason. (Shaami 2/97 – Tahtaawi Ala Maraaqil Falaah 1/402/403)

And Allah Ta’ala Knows Best