وَعَنْ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا-]
قَالَ]: «قُلْتُ لِبِلَالٍ: كَيْفَ رَأَيْتُ اَلنَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَرُدَّ عَلَيْهِمْ حِينَ يُسَلِّمُونَ عَلَيْهِ، وَهُوَ يُصَلِّي ? قَالَ: يَقُولُ هَكَذَا، وَبَسَطَ كَفَّهُ» أَخْرَجَهُ أَبُو دَاوُدَ، وََالتِّرْمِذِيُّ وَصَحَّحَهُ 11 صحيح
I asked Bilal (radiyallahu anhu), “How did you observe the Prophet (sallallahu alayhi wa sallam) replying to their (the Companions) salutation while he was engaged in prayer?” He (Bilal) said: “He used to do this way,” and he (Bilal) demonstrated by spreading his palm.
Bulugh al-Maram 267, Book 2, The Book of Prayer, Hadith 88. 1 min read