All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad
is His slave and Messenger.
The basic principle is that a person performing a voluntary prayer should perform the Prostration of Forgetfulness, just as it is prescribed for someone performing an obligatory prayer when its cause exists. Imam Al-Bukhari
said: In the Chapter (Forgetfulness in Obligatory and Voluntary Prayers): “And Ibn Abbas
performed two prostrations after his Witr prayer.”
In Musannaf Ibn Abi Shaybah
it is narrated from Abu Al-‘Aliyah Al-Bara’, may Alalah have mercy upon him, from Ibn Abbas
who said: “I saw him perform two prostrations after his Witr prayer.”
The following scholars also held the view that the prostration of forgetfulness applies in voluntary prayers: Al-Hasan Al-Basri, Sa’id ibn Jubayr, Qatadah, Al-Thawri, Malik, Al-Awza’i, Al-Shafi’i, and Ahmad ibn Hanbal
. However,Ibn Sirin did not hold this view. This was reported by Ibn al-Mundhir in Al-Ishraf.
However, some scholars believe that not every reason that requires the prostration of forgetfulness in obligatory prayer also requires it in voluntary prayer, and the detailed discussion of this is extensively explained in the books of jurisprudence.
Allah Knows best.