Forgetting one Rak`ah and not offering Sujud-ul-Sahw when making up for it

Q 3: A man joined the Zhuhr (Noon) Prayer in congregation after they stood up from the Ruku‘ (bowing) of the last Rak‘ah (unit of Prayer). After the Imam (the one who leads congregational Prayer) said the Taslim (salutation of peace ending the Prayer), the man stood up to perform a Rak‘ah and recited the first Tashahhud (a recitation in the sitting position in the second/last unit of Prayer). He then performed another two Rak‘ahs, recited the final Tashahhud, and said the Taslim. (Part No. 6; Page No. 10) A man was watching him and after he had said the Taslim, he reminded him that he had to perform another Rak‘ah and Sujud-ul-Sahw (Prostration of Forgetfulness). He, thus, stood up and performed the Rak‘ah and said the Taslim, but he did not perform Sujud-ul-Sahw. What is the ruling concerning this Salah (Prayer)? What should he have done?

A: If he did not perform Sujud-ul-Sahw intentionally, then his Salah is Batil (null and void) and he should repeat it. However, if he did not perform it mistakenly or ignorantly, then he does not have to repeat it and his Salah is correct.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.