The ruling on the prayer of one who prostrated one prostration and then prolonged sitting between the two prostrations

Question:
The questioner, or rather the questioner from Jeddah, says: Our imam prayed with us in the Fajr prayer, in the second rak'ah, O Shaykh, he prostrated one prostration and then prolonged sitting until the people thought he would conclude the prayer, so they said "Subhan Allah" to remind him, and he prostrated the second prostration, then sat for the tashahhud and concluded the prayer, but he did not prostrate for forgetfulness. What is the ruling on the prayer and the prayer of those behind him?

Answer:
It was obligatory for him to prostrate for forgetfulness, if he prolonged sitting between the two prostrations by mistake, not intentionally, but rather he forgot. So when they reminded him, he remembered and prostrated. It was obligatory for him to prostrate for forgetfulness because he added to the prayer a sitting that is not prescribed, so it was obligatory for him to prostrate. He gains [merit]. But since the interval was long and he did not prostrate, and he was ignorant of the ruling, his prayer is valid, if Allah wills, and the prayer of those behind him is valid, considering his ignorance and considering that the time for prostration was missed due to the length of the interval. Yes.