✅پاسخ :بِإِسْنَادِهِ عَنْ مُحَمَّدِ بْنِ أَحْمَدَ بْنِ یَحْیَی عَنْ أَحْمَدَ بْنِ الْحَسَنِ عَنْ عَمْرِو بْنِ سَعِیدٍ عَنْ مُصَدِّقِ بْنِ صَدَقَةَ عَنْ عَمَّارٍ فِی حَدِیثٍ قَالَ سَأَلْتُ أَبَا عَبْدِ اللَّهِ عَنْ رَجُلٍ صَلَّی ثَلَاثَ رَکَعَاتٍ وَ هُوَ یَظُنُّ أَنَّهَا أَرْبَعٌ فَلَمَّا سَلَّمَ ذَکَرَ أَنَّهَا ثَلَاثٌ قَالَ یَبْنِی عَلَی صَلَاتِهِ مَتَی مَا ذَکَرَ وَ یُصَلِّی رَکْعَةً وَ یَتَشَهَّدُ وَ یُسَلِّمُ وَ یَسْجُدُ سَجْدَتَیِ السَّهْوِ وَ قَدْ جَازَتْ صَلَاتُهُ

📚 Tahdhib, Vol. 2, Page 354

Translation: Ammar narrated in a hadith: He asked Imam Sadiq (AS) about someone who prays three rak'ahs, thinking they have prayed four, and then gives salam but realizes afterward that they only prayed three. The Imam said: Whenever they remember, they should continue the prayer, perform one more rak'ah, recite tashahhud and salam, then perform two sujud al-sahw (prostrations of forgetfulness). Their prayer is valid and correct.

In the same hadith, the Imam also said regarding someone who forgets to perform sujud al-sahw: Whenever they remember, they should perform the two prostrations of forgetfulness.