Sahih al-Bukhari, Volume 7, Book 65, Hadith 310 (USC-MSA) ⓘ
Narrated Abu Juhaifa: Allah's Messenger said, "I do not take my meals while leaning (against something).
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا مِسْعَرٌ، عَنْ عَلِيِّ بْنِ الأَقْمَرِ، سَمِعْتُ أَبَا جُحَيْفَةَ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ آكُلُ مُتَّكِئًا " .
also referenced under: Book 70, Hadith 26 (In-Book) & Hadith 5398 (DarusSalam)