Jabir reported: We were in the company of the Messenger of Allah (ﷺ), and he sent me on an errand, and when I came back (I saw him) saying prayer on his ride and his face was not turned towards Qibla. I greeted him but he did not respond to me. As he completed the prayer, he said: Nothing prevented me from responding to your greeting but the fact that I was praying.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَبُو كَامِلٍ الْجَحْدَرِيُّ، حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ كَثِيرٍ، عَنْ عَطَاءٍ، عَنْ جَابِرٍ، قَالَ كُنَّا مَعَ النَّبِيِّ صلى الله عليه وسلم فَبَعَثَنِي فِي حَاجَةٍ فَرَجَعْتُ وَهُوَ يُصَلِّي عَلَى رَاحِلَتِهِ وَوَجْهُهُ عَلَى غَيْرِ الْقِبْلَةِ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ فَلَمَّا انْصَرَفَ قَالَ " إِنَّهُ لَمْ يَمْنَعْنِي أَنْ أَرُدَّ عَلَيْكَ إِلاَّ أَنِّي كُنْتُ أُصَلِّي " .
sunnah.com