ENOfficial
It was narrated that Jabir said: “The Prophet (ﷺ) sent me on an errand, then I caught up with him while he was performing prayer, and I greeted him. He gestured to me, then when he finished, he called me and said: ‘You greeted me before, but I was performing prayer.’”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ الْمِصْرِيُّ، أَنْبَأَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ بَعَثَنِي النَّبِيُّ ـ صلى الله عليه وسلم ـ لِحَاجَةٍ ثُمَّ أَدْرَكْتُهُ وَهُوَ يُصَلِّي فَسَلَّمْتُ عَلَيْهِ فَأَشَارَ إِلَىَّ فَلَمَّا فَرَغَ دَعَانِي فَقَالَ " إِنَّكَ سَلَّمْتَ عَلَىَّ آنِفًا وَأَنَا أُصَلِّي " .
sunnah.com