Answered by Shaykh Faraz Rabbani
So, if I made this mistake as an imaam, is sajdah sahw necessary? If I did not perform sajdah sahw during the two instances this has occurred, then shall I make up the prayer and tell my dad (who prayed with me) to make it up as well?
1. It is necessary (wajib) for the imam to recite quietly in the silent prayers [i.e. Zuhr and Asr]. If he recites out loud, this will entail leaving a necessary (wajib) act. [Ala al-Din Abidin, Gifts of Guidance]
2. If a necessary (wajib) act is forgetfully omitted, it is necessary to perform 2 forgetfulness prostrations at the end of the prayer. [SEE: (http://www.sunnipath.com) for details]
3. If a necessary act is intentionally omitted, one must repeat one’s prayer within the time. [For details, see article below.]
4. If a necessary act is forgetfully omitted but one failed to perform the forgetfulness prostrations, one must repeat the prayer within the time. [Gifts]
5. After the time, it would be best to tell your father to repeat the prayer if he is practicing and committed; otherwise, you shouldn’t.