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 facing the east or the west. I greeted him with salam and he gestured to me. Then when he finished he called me and said: "O Jabir!" So I came and said: "O Messenger of Allah, I greeted you with Salam but you did not answer." He said: 'I was praying.'
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مُحَمَّدُ بْنُ هَاشِمٍ الْبَعْلَبَكِّيُّ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ شُعَيْبِ بْنِ شَابُورٍ، عَنْ عَمْرِو بْنِ الْحَارِثِ، قَالَ أَخْبَرَنِي أَبُو الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ بَعَثَنِي النَّبِيُّ صلى الله عليه وسلم فَأَتَيْتُهُ وَهُوَ يَسِيرُ مُشَرِّقًا أَوْ مُغَرِّبًا فَسَلَّمْتُ عَلَيْهِ فَأَشَارَ بِيَدِهِ ثُمَّ سَلَّمْتُ عَلَيْهِ فَأَشَارَ بِيَدِهِ فَانْصَرَفْتُ فَنَادَانِي " يَا جَابِرُ " . فَنَادَانِي النَّاسُ يَا جَابِرُ . فَأَتَيْتُهُ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنِّي سَلَّمْتُ عَلَيْكَ فَلَمْ تَرُدَّ عَلَىَّ . فَقَالَ " إِنِّي كُنْتُ أُصَلِّي " .
sunnah.com