Sunan Ibn Majah Book 5, Hadith 1018

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 ‏"‏ ‏.‏

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.’”