Sunan Ibn Majah, Hadith 1019 ⓘ
It was narrated that ‘Abdullah said: “We would greet others during the prayer, and it was said to us: ‘During the prayer one is preoccupied.’”
حَدَّثَنَا أَحْمَدُ بْنُ سَعِيدٍ الدَّارِمِيُّ، حَدَّثَنَا النَّضْرُ بْنُ شُمَيْلٍ، حَدَّثَنَا يُونُسُ بْنُ أَبِي إِسْحَاقَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا نُسَلِّمُ فِي الصَّلاَةِ فَقِيلَ لَنَا إِنَّ فِي الصَّلاَةِ لَشُغْلاً .
also referenced under: Book 5, Hadith 1072 (In-Book) & Volume 2, Book 5, Hadith 1019 (DarusSalam)