Question Summary:
Forgetting number of Rakaat in Sunnah and Nafl Salah

Question Detail:

Assalamualaikum,
 
I wanted to ask if sajdah sahw is required for forgetting the number of rakats performed in sunnah and nafl prayer?
jazakhalah khair

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
There are a few scenarios pertaining to the one who forgets which Rakah he is in;
1)     If this is the first time occurring, he will restart the Salah.[1]
2)     If this a multiple occurrence, he will continue based on whichever Rakah he is more inclined towards (ghalib zan).
For example, if one has doubt whether he is in the third Rakah or fourth and his inclination is towards him being in the third Rakah, so he will continue his Salah based on that inclination. 
In this scenario, if one occupies the amount of time for three tasbih figuring out which Rakah he is in, he will do Sajdah Sahw.
3)     If he is uncertain of his inclination, he should continue based on what is certain (the lesser amount).
In this case, he will sit for tashahhud in each Rakah which he thinks may be the last Rakah so as not to accidently miss the fardh of sitting in the last tashahhud.[2]
Also in this scenario, one will do Sajdah Sahw at the end of his Salah.[3]
The examples for these scenarios have been described in the following Hadith;
عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ، قَالَ: سَمِعْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: إِذَا سَهَا أَحَدُكُمْ فِي صَلاَتِهِ فَلَمْ يَدْرِ وَاحِدَةً صَلَّى أَوْ ثِنْتَيْنِ فَلْيَبْنِ عَلَى وَاحِدَةٍ، فَإِنْ لَمْ يَدْرِ ثِنْتَيْنِ صَلَّى أَوْ ثَلاَثًا فَلْيَبْنِ عَلَى ثِنْتَيْنِ، فَإِنْ لَمْ يَدْرِ ثَلاَثًا صَلَّى أَوْ أَرْبَعًا فَلْيَبْنِ عَلَى ثَلاَثٍ، وَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ. هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ.
Translation: Rasulullah (صلي الله عليه وسلم) said, “When one of you becomes forgetful in his Salah and he does not know if he prayed one Rakah or two, he should consider it to be one. When he does not know if he prayed three or four, he should consider it to be three, and he should perform Sajdah (Sahw) before he says Salam.”[4]
 
And Allah Ta’āla Knows Best
Saad Haque
Student Darul Iftaa
New Jersey, USA 
Checked and Approved by,
Mufti Husain Kadodia.