بُلُوغُ الْمَرَامِ
Bulugh al-Maram 225
The Book of Prayer
وَعَنْ اِبْنِ عُمَرَ - رَضِيَ اَللَّهُ عَنْهُمَا-]قَالَ] : { قُلْتُ لِبِلَالٍ : كَيْفَ رَأَيْتُ اَلنَّبِيَّ - صلى الله عليه وسلم -يَرُدَّ عَلَيْهِمْ حِينَ يُسَلِّمُونَ عَلَيْهِ , وَهُوَ يُصَلِّي ? قَالَ : يَقُولُ هَكَذَا , وَبَسَطَ كَفَّهُ } أَخْرَجَهُ أَبُو دَاوُدَ , وَاَلتِّرْمِذِيُّ وَصَحَّحَهُ [1]
Narrated Ibn 'Umar (RA): I asked Bilal (RA), "How did you observe the Prophet (ﷺ) 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. [Reported by Abu Dawud and at-Tirmidhi. The latter graded it Sahih (authentic)].
Sunnah.com reference: Book 2, Hadith 90