Sahih al-Bukhari 5399
Sahih al-Bukhari · Book 70: Food, Meals · To eat while leaning
Narrated Abu Juhaifa:
حَدَّثَنِي عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، أَخْبَرَنَا جَرِيرٌ ، عَنْ مَنْصُورٍ ، عَنْ عَلِيِّ بْنِ الأَقْمَرِ ، عَنْ أَبِي جُحَيْفَةَ ، قَالَ كُنْتُ عِنْدَ النَّبِيِّ صلى الله عليه وسلم فَقَالَ لِرَجُلٍ عِنْدَهُ " لاَ آكُلُ وَأَنَا مُتَّكِئٌ " .
While I was with the Prophet (ﷺ) he said to a man who was with him, "I do not take my meals while leaning."