Musannaf Ibn Abi Shaybah

Hazrat Sha'bi and Hazrat Sa'id bin Jubair say that Sajdah Sahw is performed in Nafl (voluntary) prayers.

Hasan believed in performing Sajdah Sahw in Nafl prayers.

Hazrat Saeed bin Musayyib says that just as there is sujood al-sahw in obligatory prayers, similarly there is sujood al-sahw in voluntary prayers as well.

Ibn Aun says that I asked Muhammad whether doubt (uncertainty) is considered in voluntary (nafl) prayers. He replied, "I do not know what its status is." I asked, "In the case of doubt in nafl, should two prostrations of forgetfulness (sajda sahw) be performed?" He replied, "Do you consider nafl to be similar to obligatory (fard) prayers? If such a situation happened to me, I would not perform sajda sahw."

Hazrat Qatadah did not consider doubts in Nafl (voluntary) prayers.