General Behavior (Kitab Al-Adab)
Dawud :: Book 41 : Hadith 4830
Narrated Ash-Sharid ibn Suwayd:
The Apostle of Allah (peace_be_upon_him) came upon me when I was sitting thus: having my left hand behind my back and leaning on the fleshy part of it, and said: Are you sitting in the manner of those with whom Allah is angry?