In Sajdah, it is necessary that the face (the forehead and the nose), both the palms, both knees and both feet touch the ground. Out of these, the forehead and at least 1 finger of any foot is Fardh (obligatory).
The meaning of the word Sajdah is to bow with humility and to touch forehead on the ground. The Sajdah is to respect and bow to the greatness of the Lord Almighty Allah (Subhanahu Wa Ta'ala) and lifting both legs off the ground is more akin to some kind of play than respect, which is against the spirit of respect. As given in Radd Ul Muhtaar, Vol. 1, Pg. No: 330.
Imam Rafa'i (May Allah Ta’ala shower His mercy on him) has written in At Tahreer Al Mukhtaar Li Radd Ul Muhtaar:
Touching 1 finger of any foot is enough to complete the Fardh (obligation), as if in Sajdah, if both the feet are lifted off the ground and not even 1 finger of any foot is not on the ground then the Sajdah will not be correct as per the Shariah and the Salaat will be rendered invalid (Batil in Urdu). (At Tahreer Al Mukhtaar Li Radd Ul Muhtaar, Vol. 1, Pg. No: 56)
If the place where the Sajdah is being offered such that the forehead touches the ground and the head does not go down too much then the Sajdah is correct and with it the Salaat is also valid. On the other hand, if the place where the Sajdah is being offered so soft that the forehead does not touch the ground and the head keeps bobbing, then the Sajdah (and consequently the Salaat) is also not correct. However, if one feels the hardness of the surface while prostrating and the head touches the ground and does not bob or move, then the Sajdah is correct and valid even on grass.
It is given in Radd Ul Muhtaar, Vol. 1, Pg. No: 370:
Translation: It is necessary that while performing the Sajdah, one should feel the hardness of the surface, which means that when the person offers the Sajdah nicely then his (or her) forehead should not go down more than where it was placed. For this reason, Sajdah is not correct on a soft surface like a grass or a quilt. If one feels the hardness of the surface, then it is correct.
And Allah (Subhanahu Wa Ta’ala) knows best.
Mufti Syed Ziauddin Naqshbandi Qadri
[Associate Professor of Islamic Law, Jamia Nizamia,
Founder/Director, Abul Hasanaat Islamic Research Center]