It was narrated that Jabir said:

"The Messenger of Allah ﷺ 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 ﷺ 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