As you asked me to quote from Ibn Uthaymeen رحمه الله here it is: "ولا تجلس متكئا فهذا سوء أدب " شرح حلية طالب العلم صفحة ١١٧ And before that it talks about putting our legs between our hands, so does this all apply to outside of the lecture too? جزاكم الله خيرا أستاذنا

Ustadh AbdulAziz al-Haqqan

This did not mention puttin your back against the wall.