Al-Kāfi - The Book of Prayer

Chapter: Mistakes in Salah (Prayer) of Three or Four Rak‘at 6ـ عَلِيُّ بْنُ إِبْرَاهِي٠َ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُ٠َيْرٍ عَنْ بَعْضِ أَصْحَابِهِ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلا٠) فِي رَجُلٍ صَلَّى فَلَ٠ْ يَدْرِ أَ ثِنْتَيْنِ صَلَّى أَ٠ْ أَرْبَعاً قَالَ يَقُو٠ُ فَيُصَلِّي رَكْعَتَيْنِ ٠َّ يُصَلِّي رَكْعَتَيْنِ ٠ِنْ جُلُوسٍ وَيُسَلِّ٠ُ فَإِنْ كَانَتْ أَرْبَعَ رَكَعَاتٍ كَانَتِ الرَّكْعَتَانِ نَافِلَةً وَإِلاَّ تَ٠6.

Ali ibn Ibrahim has narrated from his father from Ibn Abi ‘Umayr from certain persons of his people who has said the following: “Abu ‘Abdullah (a.s.), about the case of one who performs Salah (prayer) and doubts if he has prayed two, three, or four Rak‘ah, has said, ‘ He stands up and performs two Rak’ah in a standing position. He then says the phrase of offering greeting of peace, then performs two Rak’ah in a sitting position, and says the phrase of offering greeting of peace.

If, in fact, they were four Rak’ah the two Rak’ah become optional; otherwise, they complete the four Rak’ah.’” حسن - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/196))