Assalamualaikum ustaz. Is it necessary for the makmum to perform sahwi prostration if the imam didn’t perform it? For example, the imam missed the qunut for Subuh and didn’t perform the sahwi prostration or intentionally leave it. Is it obligatory for us to perform the sahwi prostration after the imam gives the salam?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
According to the book al-Fiqh al-Manhaji: All-Sahwu is a deficiency that happens to a person who’s praying. Whether it happens intentionally or because he forgets. And prostration – at the end of prayer – mending or compensating for the deficiency. 
Sahwi prostration is based on a famous story of Zul Yadain, narrated by Abu Hurairah RA, he said:
صَلَّى بِنَا النَّبِيُّ صلى الله عَلَيْهِ وَسَلَّم الظُّهْرَ أَو الْعَصْرَ ، فَسَلَّم ، فَقَالَ لَهُ ذُو الْيَدَينِ : الصَّلَاةُ يَا رَسُولَ اللَّهِ ، أَنقصت ؟ فَقَال النبيُّ صلى الله عليه وسلم : أَحَقٌّ مَا يَقُول ؟ قالوا نعمْ ، فصلَّى ركْعَتَينِ أُخْرَيَتين ، ثمَّ سَجَدَ سَجْدَتَين
“O Allah’s Messenger (ﷺ)! Has the prayer been reduced?” The Prophet (ﷺ) asked his companions in the affirmative. So, Allah’s Messenger (ﷺ) I offered two more rak`at and then performed two prostrations (of Sahu).” 
Among the reasons that call for the sahwi prostration are: A person who leaves one of the ab’adh sunnah in prayer. Ab’adh sunnah in prayer is as the following:
- First Tasyahhud
- Praise and salutation to the Prophet PBUH after the first tasyahhud
- Sit for the first tasyahhud
- Praise and salutations to the family of the Prophet PBUH after the final tasyahhud
- Qunut during the I’tidal in the second rakaat of Subuh prayer
Among the evidences which strengthened it is a narration from Al-Mughirah bin Syu’bah, he said, Rasullullah PBUH said:
إِذَا قَامَ أَحَدُكُم مِنَ الرّكْعَتَينِ فَلَمْ يَسْتَتِمَّ قَائِمًا فَلْيَجْلِسْ ، وَإِذَا اسْتَتَمَّ قَائِمًا فَلَا يَجْلِسْ ، وَيَسْجُدْ سَجْدَتَي السَّهْوِ
“The Messenger of Allah (ﷺ) said: ‘If anyone of you stands after two Rak’ah, if he has not yet stood up fully, let him sit down again, but if he has stood up fully, then let him not sit down, and let him perform two prostrations for forgetfulness (Sahw).’” 
According to the above explanations and arguments presented, if an imam missed or intentionally leave qunut recitation during Subuh prayer, it is sunnah for him to perform sahwi prostration. However, if he didn’t perform it, it doesn’t invalidate the prayer. The reason is sahwi prostration is sunnah when certain reasons previously stated happened.
While for a makmum, it is an obligation for him to follow his imam in every movement of the prayer. If the imam performs sahwi prostration, he should also do so. However, if the imam didn’t perform the sahwi prostration (when there is reason to do so), then it isn’t necessary for him to perform it after the imam gives the salam.
 See al-Fiqh al-Manhaji, 1/172
 Narrated by al-Bukhari (1169)
 Narrated by Ibn Majah (1208)