From Sama’ah, from Abi Abdallah (a):,,I asked him about a man, to whom the peace [Salaam] is given and he is in prayer.” He (a) said: “He answers with {Salaam alaikum} and he does not say {Wa alaikum salam}, for verily the Messenger of God (s) was in prayer when ‘Ammar ibn Yasir passed by him and ‘Ammar gave the Salaam upon him, so the Prophet (s) answered him in the same way.” [Al-Kafi, vol. 3, page 208, hadith 1]

مُحَمَّدُ بْنُ يَعْقُوبَ عَنْ مُحَمَّدِ بْنِ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عُثْمَانَ بْنِ عِيسَى عَنْ سَمَاعَةَ عَنْ أَبِي عَبْدِ اللَّهِ ع قَالَ سَأَلْتُهُ عَنِ الرَّجُلِ يُسَلَّمُ عَلَيْهِ وَ هُوَ فِي الصَّلَاةِ قَالَ يَرُدُّ سَلَامٌ عَلَيْكُمْ وَ لَا يَقُولُ وَ عَلَيْكُمُ السَّلَامُ فَإِنَّ رَسُولَ اللَّهِ ص كَانَ قَائِماً يُصَلِّي فَمَرَّ بِهِ عَمَّارُ بْنُ يَاسِرٍ فَسَلَّمَ عَلَيْهِ عَمَّارٌ فَرَدَّ عَلَيْهِ النَّبِيُّ ص هَكَذَا

Mansur ibn Hazim, from Abi ‘Abdallah (a) said: ,,If someone gives you the Salaam and you are praying. He (a) said: “Answer him quietly like he has said.” [Tahdhib al-Ahkam, Volume 2, page 356, Hadith 1366]

وَ بِإِسْنَادِهِ عَنْ سَعْدٍ عَنْ مُحَمَّدِ بْنِ عَبْدِ الْحَمِيدِ عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ بْنِ بَزِيعٍ عَنْ عَلِيِّ بْنِ النُّعْمَانِ عَنْ مَنْصُورِ بْنِ حَازِمٍ عَنْ أَبِي عَبْدِ اللَّهِ ع قَالَ إِذَا سَلَّمَ عَلَيْكَ الرَّجُلُ وَ أَنْتَ تُصَلِّي قَالَ تَرُدُّ عَلَيْهِ خَفِيّاً كَمَا قَالَ