10 Situations where Sajdah Sahw is necessary (Wajib)
If a Wajib act of Salah is forgetfully left out, the compensatory prostrations (Sajdah Sahw) must be performed. (Sharh-ul-Wiqayah, vol. 1, p. 220)
Although there are many situations where Sajdah Sahw is required (Wajib), here are ten of the most common:
1. Sajdah Sahw is required if you miss out even one verse of Surah Al-Fatihah or recite Surah Al-Fatihah twice before reciting another Surah in:
a. The first two cycles (Rak’at) of a Fard Salah or;
b. Any cycle of a supererogatory or;
c. Any cycle of a Wajib Salah, (Durr-e-Mukhtar, vol. 2, p. 656)
2. Sajdah Sahw is required if you forget to perform Ta’deel-e-Arkaan, (Fatawa Hindiyyah, vol. 1, p. 127).
3. If you forget to recite Tashahhud (اَلتَّحِیَّات) but remember immediately after giving Salaam, then you should remain seated and recite Tashahhud and then perform Sajdah Sahw. (Fatawa Hindiyyah, vol. 1, p. 127)
4. Similarly, if you recited Surah Al-Fatihah instead of Tashahhud, Sajdah Sahw is required to compensate this error. (Fatawa Hindiyyah, vol. 1, p. 127)
5. Sajdah Sahw is required if you forget to recite Du’a Qunoot during the Witr Salah. (Fatawa Hindiyyah, vol. 1, p. 111)
6. If you forget to recite the Takbeer before Du’a Qunoot, Sajdah Sahw will be required. (Bahar-e-Shari’at, vol. 1, p. 714)
7. If you are praying a silent prayer (in which silent recitation is necessary, such as Zuhr and ‘Asr) alone but forgetfully recite loudly, then Sajdah Sahw will be required. (Rad-ul-Muhtar, vol. 2, p. 657)
8. Sajdah Sahw will be required if you recite Tashahhud after Surah Al-Fatihah while standing during the first two cycles of Salah. (Fatawa Hindiyyah, vol. 1, p. 127)
9. If you miss out any part of Tashahhud during Qa’dah, Sajdah Sahw will be required. (Fatawa Hindiyyah, vol. 1, p. 127)
10. If you recite ‘اَللّٰھُمَّ صَلِّ عَلٰی مُحَمَّدٍ’ after Tashahhud during Qa’dah Oola Sajdah Sahw will be required, (Durr-e-Mukhtar wa Rad-ul-Muhtar, vol. 2, p. 657)
Note: Sajdah Sahw is required in all of the aforementioned scenarios only if the specified wajib acts are left out forgetfully. If one intentionally leaves any of them out, then Sajdah Sahw is not enough and the Salah must be repeated.
May Allah Almighty give us the ability to offer the daily prayers with sincerity whilst observing all the acts that are Fard, Wajib, Sunnah and Mustahab.
اٰمِیْن بِجَاہِ النَّبِیِّ الْاَمِیْن صَلَّی اللہ تَعَالٰی عَلَیْہِ وَاٰلہٖ وَسَلَّم
(Jami’ah-tul-Madinah Faizan-e-‘Aalam Shah Bukhari, Karachi)
 Ta’deel-e-Arkaan refers to remaining in Ruku, Sajdah, Qawmah and Jalsah for at least the duration in which ‘سُـبْحٰـنَ الـلّٰــه’ can be uttered once.
 Qa’dah Oola refers to sitting in Salah after the first two cycles of Fard, Witr and Sunnah Muakkadah.