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?
Answer:

Walaikum assalam,
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.
Wassalam,
Faraz Rabbani
