سَأَلَ مُحَمَّدُ بْنُ مُسْلِمٍ أَبَا جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلرَّجُلِ يُسَلِّمُ عَلَى اَلْقَوْمِ فِي اَلصَّلاَةِ فَقَالَ «إِذَا سَلَّمَ عَلَيْكَ مُسْلِمٌ وَ أَنْتَ فِي اَلصَّلاَةِ فَسَلِّمْ عَلَيْهِ تَقُولُ اَلسَّلاَمُ عَلَيْكَ وَ أَشِرْ بِإِصْبَعِكَ.
Muhammad ibn Muslim asked Imam Muhammad ibn Ali Al-Baqir (a.s) about a person who greets others while in prayer. Imam (a.s) said: "If a Muslim greets you while you are in prayer, respond to him by saying, ‘As-salamu ‘alayka,' and gesture with your finger."