The obligation of one who says the closing salam before completing his prayer

Honorable Shaykh! A man is praying with the congregation and he doubts about his prayer, whether he prayed three or four rak'ahs. He leans toward the opinion that he prayed four, so he prostrates for forgetfulness and says the closing salam.Then the ma'mum (those praying behind the imam) said to him: [He doubts] that he prayed three and stood up to perform a fourth! So does he prostrate for forgetfulness a second time? Or is the first prostration for forgetfulness sufficient?

We say: It is obligatory for him to prostrate for forgetfulness after the salam from the fourth rak'ah; because he said the salam before completing his prayer, and saying the salam before completing the prayer necessarily involves forgetfulness, and the prostration is to be done after the salam.