Sunan an-Nasa'i, Volume 2, Book 13, Hadith 1190 (DarusSalam) ⓘ

It was narrated that Jabir said: "The Messenger of Allah sent me on an errand then I came back to him while he was praying. I greeted him with the salam and he gestured to me. When he finished he called me and said: 'You greeted me with Salam just now and I was praying.' And he was facing toward the east that day."

أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ بَعَثَنِي رَسُولُ اللَّهِ صلى الله عليه وسلم لِحَاجَةٍ ثُمَّ أَدْرَكْتُهُ وَهُوَ يُصَلِّي فَسَلَّمْتُ عَلَيْهِ فَأَشَارَ إِلَىَّ فَلَمَّا فَرَغَ دَعَانِي فَقَالَ ‏ "‏ إِنَّكَ سَلَّمْتَ عَلَىَّ آنِفًا وَأَنَا أُصَلِّي ‏" ‏ ‏.‏ وَإِنَّمَا هُوَ مُوَجَّهٌ يَوْمَئِذٍ إِلَى الْمَشْرِقِ