The Pillars and the Obligatory Acts of the Prayer
The pillars (arkaan; singular: rukn) of the prayer are its essential parts that are necessary for its validity. If any of these pillars is left out due to forgetfulness or deliberate omission, the prayer becomes invalid.
They are as follows:
The opening takbeer; assuming the standing position, if one has the ability to do so; reciting Soorat Al-Faatihah (in the case of performing the prayer alone, i.e., not being led by an imaam); bowing; rising from the bowing position; prostrating, sitting between the two prostrations, the final tashahhud, sitting while reciting the tashahhud; the tasleem.; and making certain that each body part assumes the appropriate position before one moves from one position to another.
The obligatory acts of the prayer (waajibaat; singular: waajib)are those acts that must be done in the prayer. If they are deliberately omitted, the prayer becomes invalid. However, if they are left out due to inattention or forgetfulness, the prayer is still valid, but the acts that are left out can be compensated for with the two prostrations of forgetfulness, as will be explained later.
The obligatory acts of the prayer are as follows:
Recitation of takbeer throughout the prayer other than the opening takbeer; saying Subhaana rabbiy-al-‛adheem (Glory be to my Lord, the Almighty) once; saying Sami‛allaahu li man hamidah (Allah listens to him who praises Him), whether one is praying alone or one is the prayer leader (imaam); saying Rabbanaa wa lakal-hamd (Our Lord, to You is due all praise); Subhaana rabbiyal-a‛laa (Glory be to my Lord, Most High) once; saying Rabbighfir lee, (O Lord, forgive me) while sitting between the two prostrations once; and reciting the first tashahhud. These obligatory acts can be made up for with the two prostrations of forgetfulness.
The recommended acts of the prayer (sunan as-salaat) are those words and acts other than the pillars nor the obligatory acts of the prayer. They serve to complement the prayer and it is recommended to observe them. Omission of any of these acts, however, does not render the prayer invalid.