Sunan an-Nasa'i, Volume 2, Book 13, Hadith 1189 (DarusSalam) ⓘ
It was narrated from 'Ammar bin Yasir that: He greeted the Messenger of Allah with the Salam when he was praying, and he returned the greeting.
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا وَهْبٌ، - يَعْنِي ابْنَ جَرِيرٍ - قَالَ حَدَّثَنَا أَبِي، عَنْ قَيْسِ بْنِ سَعْدٍ، عَنْ عَطَاءٍ، عَنْ مُحَمَّدِ بْنِ عَلِيٍّ، عَنْ عَمَّارِ بْنِ يَاسِرٍ، أَنَّهُ سَلَّمَ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم وَهُوَ يُصَلِّي فَرَدَّ عَلَيْهِ .
also referenced under: Hadith 1188 (Reference) & Book 13, Hadith 10 (In-Book)