In the times when confusion comes in my salah, sometimes there is doubt that I have left a rakat and sometimes I am sure that I have left a rakat. I have to do sajdah sahu in second case but, what about the first case ? I have to do sajda sahu in first case also ?
There are three reasons for which one is to do the prostration of forgetfulness.
- Unintentionally leaving something which is recommended or obligatory to do in the salah such as reciting ayat or surahs after Surah Al Fatiha (recommended), or not do a rukoo' after Al Fatiha (obligatory).
- Or unintentionally making something extra which is not part of the sunnah such as making a fifth rakah in 'Isha salah
- Or being uncertain. This includes not knowing if you are in the third or fourth rakah, for example.
If you add or take something of the salah intentionally, then your salah will become invalid. So you should do the prostration of forgetfulness in the first case.
If your inclination is towards having missed a rakah then pray and complete. if your inclination is towards having prayed correctly, then move on. One may, however, do sajda sahu should his/her inclination be wrong. For any excess, there is really no need of sajda sahu it is considered as an additional blessing.