Shama-il Tirmidhi Chapter 021, Hadith Number 003 (125).
Abu Juhayafah radiyallahu anhu reports that: "Rasulullah sallallahu alaihe wasallam said: 'I do not lean when I have meals.'"
This manner of eating is against humility, and it drives one to eat more. Soem people are of the opinion that it results in creating a large tummy, and the food also does not digest quickly. Sayyidina Rasulullah sallallahu alaihe wasallam mentions himself so that he may be emulated. The Ulama write that there are four methods of leaning, and all four are included here. The first is to lean with either side of the body on a wall, pillow, etc. The second is to lean with the palms on the ground. The third is to sit cross legged on a pillow. The fourth method is to lean with the back on a large pillow or wall. All these postures are included in the above prohibition.