What is the ruling on the prayer of one who prayed 'Isha and, forgetting, recited the Qunoot supplication in the second rak'ah, thinking it was the fourth, then completed his prayer and performed the prostration of forgetfulness (sujood as-sahw) at the end of it? And is the prostration of forgetfulness valid in this case?

Scholars have differed regarding the ruling on the prostration of forgetfulness (sujud al-sahw) for someone who inadvertently recites a prescribed ذكر (remembrance/invocation) during prayer in an improper place, such as reciting Qur'an during bowing (rukūʿ) or prostration (sujūd). Some scholars hold that the prostration of forgetfulness is not prescribed in this case, while others view its prescription as recommended (mustahabb) but not obligatory (wājib), and the prayer remains valid.