Qa’dah al-akhirah is the sitting after the final sajdah of every salah.  Men spread the left foot and raise the right one in this sitting. (Jami’ at-Tirmidhi, Salah, 102) Women on the other hand spread their feet to the right and sit on their left sides.  Sitting for a duration in which one can recite the tashahhud, i.e. tahiyyah, is necessary for the salah's validity.  After emphasizing that all expressions of exaltedness and all worship is exclusively for Allah (swt) in the first part of the du'a, one prays that His mercy and blessings is upon the prophets; and that peace and well-being is upon all righteous servants. One continues with the sunnah du'as called Salli and Barik after testifying, in the final part of the tahiyyah, that Allah (swt) is the only god and that Muhammad (saw) is His servant and messenger.  (Sahih Bukhari, Adhan, 148; Jami’ at-Tirmidhi, Da’awat, 64) Finally, he greets to the right and left by saying “as-salamu alaikum wa rahmatullah”  (Allah’s peace and mercy be upon you). Thus, the Muslim who performs salah ends his private communication with his Lord until the next meeting and returns to his daily work. (Sunan an-Nasai, Sahw, 70; Sunan abu Dawud, Salah, 183, 184)

Besides the fardh actions of salah, which we explained as pillars and prerequisites, there are several actions that are necessary (wajib) to perform.  Both fardh and wajib acts are equally required, but there is a difference: The salah does not become invalid when a wajib act is left out accidentally. Performing the sajdah al-sahw (prostration of forgetfulness) at the end of the salah compensates for such a mistake. 

One of the most important wajib acts of salah is ta’dil al-arkan.  It is to perform the pillars of the salah properly, giving each one its due right. Just as Prophet Muhammad (saw) indicated in His saying, “The worst kind of theft is to steal from prayer” (Muwatta’, Qasr as-salah, 23), ta’dil al-arkan is to perform the salah completely. Therefore, a believer is should not hurry and wait for at least a duration in which he can say “subhanallah” during ruku’, the standing after ruku’, in sajdah and during the sitting in between the two sajdahs. 

The wajib acts of salah are to start the salah by saying “Allahu akbar”; to recite an extra surah – or some verses similar in length- after surah al-Fatiha in the first two rak’ah of the fardh prayers; to recite silently in the noon and afternoon prayers, and in all supererogatory prayers during the day; to place the forehead along with the nose on the ground during sajdah; to sit after the second rak’ah in prayers with three or four rak’ah; to say the tashahhud in the first and final sitting.