a Wajib is missed forgetfully during the Salah, and if a Wajib is missed deliberately, then repeating the Salah becomes Wajib.

Moreover, it has also been learnt that it is Wajib to offer Salah calmly because that elderly man came after offering Salah hastily, therefore he was asked to repeat his Salah; that is, every time when he would come after offering Salah, he would say Salam and would be asked to go back.

The Blessed and Beloved Rasool صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم did not teach him the method of Salah the very first time, but rather he صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم taught him the method after making him offer Salah many times so that he could remember this event and he could memorize this ruling very well because the thing we get with great labour and wait, settles in the mind.

(Mirat-ul-Manajih, vol. 2, pp. 12)

Dear Islamic sisters! From the blessed Hadees and its commentary it has become obvious that we should offer Salah with extreme calmness without any haste. It has been stated in the blessed Hadees that: ‘Perform Ruku’ with calmness then stand up straight raising your head from Ruku’, then perform Sajdah with calmness, then sit calmly raising your head from sajdah’, this is called Ta’deel-e-Arkaan.

‘Allamah Maulana Mufti Muhammad Amjad ‘Ali A’zami رَحْمَةُ اللهِ تَعَالٰی عَلَيْه has stated in Bahar-e-Shari’at: Maintaining Ta’deel-e-Arkaan, i.e. staying in Ruku’, Sujood, Qawmah [standing after Ruku’] and Jalsah [sitting between two Sajdahs] for the amount of time in which ‘سُبْحٰنَ اﷲ’ can be uttered at least once, is Wajib. Sajdah Sahw becomes Wajib if someone forgets Ta’deel-e-Arkaan.

(Bahar-e-Shari’at, vol. 1, pp. 518, Part. 4, pp. 711)

Anyhow, one should have information about the Wajibat, Makruhaat, Mufsidaat (The acts which invalidate Salah) of Salah, and other necessary rulings along with the correct method of performing Wudu and Ghusl (the ritual bath) so that our Salahs do not become invalid because the way offering Salah is obligatory for every adult and sane Muslim, similarly, it is also obligatory to learn rulings on Salah relevant to circumstances.

(Bahar-e-Shari’at, part 3, vol. 1, pp. 518; part 4, vol. 1, pp. 711)