بسم الله الرحمن الرحيم
In the Name of Allah The most Gracious The Most Merciful
In the wajib (obligatory) elements of the prayer if one forgets them then his prayer is still valid but if one misses an element on purpose then the prayer is not valid. If one forgets one of the obligations and rememberes later on that they forgot it then they do not have to repeat the action but at the end of the prayer they must perform the prostration of forgetfulness.
1. All the takbir’s except the opening takbir
Narrated ‘Abdullah bin ‘Umar: I saw Allah’s Apostle opening the prayer with the Takbir and raising his hands to the level of his shoulders at the time of saying the Takbir, and on saying the Takbir for bowing he did the same; and when he said, “Sami’a-Allahu Liman hamidah”, he did the same and then said, “Rabbana wa lakal-hamd.” But he did not do the same on prostrating and on lifting the head from it.” (Sahih al-Bukhari, Book: Characteristics of Prayer, Hadith 705)
2. Saying “Subhaana Rabbiyal-‘Adheem” when bowing
…he bowed and he paused in bowing as long as he stood up; he said while bowing, “Glory be to my mighty Lord, (Subhana rabiyal-‘adheem” “Glory be to my mighty Lord” ; then he raised his head…” (Abu dawood, Book: Kitab as salat (prayer), Hadith 0873)
3. Saying “Sami’a-Allahu Liman Hamidah”: This applies to the one leading the
prayer and the one praying alone.
4. Saying “Rabbanaa wa Lakal-Hamd”: This applies to everyone praying
This hadith is evidence for the two above obligations: Narrated Abu Huraira: When the Prophet said, “Sami’-Allahu Liman hamida,” (Allah heard those who sent praises to Him), he would say, “Rabbana wa-laka-l-hamd.” On bowing and raising his head from it the Prophet used to say Takbir. He also used to say Takbir on rising after the two prostrations. (Sahih al-bukhari, Book of Characteristics of Prayer, Hadith 761)
5. Saying “Subhaana Rabby al-A’alaa” when prostrating
…”he said while prostrating: “Glory be to my most high Lord” (Subhaana rabiyal-a’la); then he raised his head after prostration, and sat as long as he prostrated…”(Abu dawood, Book: Kitab as salat (prayer), Hadith 0873).
6. Saying “Rabbi-Ighfir-Lee” while in between the two prostrations
…he raised his head after prostration, and sat as long as he prostrated, and said while sitting: “O my Lord forgive me.”… (Abu dawood, Book: Kitab as-salat (prayer), Hadith 0873)
7. The first Tashahhud
8. Sitting during the first Tashahhud
The final two obligations are shown in this hadith: …At the end of every two rak’ahs he recited the tahiyya (greetings to the prophet)… (Sahih muslim, Book of prayers, Hadith 1014)