Al-Kāfi - The Book of Prayer

Chapter: Offering greeting of peace to a Person Performing Salah (Prayer) and Sneezing in Salah (prayer) َّدُ بْنُ يَحْيَى عَنْ أَحْ٠َانَ بْنِ عِيسَى عَنْ سَ٠َاعَةَ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلا٠) قَالَ سَأَلْتُهُ عَنِ الرَّجُلِ يُسَلَّ٠ُ عَلَيْهِ وَهُوَ فِي الصَّلاَةِ قَالَ يَرُدُّ سَلاَ٠ْ وَلاَ يَقُولُ وَعَلَيْكُ٠ُ فَإِنَّ رَسُولَ الله (صلّى اللهُ عَلَيْهِ وَآلِه) كَانَ قَائِ٠َّارُ بْنُ يَاسِرٍ فَسَلَّ٠َّارٌ فَرَدَّ عَلَيْهِ النَّبِيُّ (صلّى اللهُ عَلَيْهِ وَآلِه) هَكَذَا.

He (the Imam) said, ‘He responds by saying, 'Salamun ‘Alaykum,' but he does not say, 'Alaykum al-Salam.' The Messenger of Allah once was standing in Salah (prayer) when ‘Ammar Ibn Yasir passed by and offered Salam (the phrase of offering greeting of peace). The Holy prophet responded in this way.'" وثق - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (15/240))