Ibn al-Qayyim رحمه الله said:
“A person’s Adab is a sign of his happiness and success. The lack of Adab is a sign of his misery and ruin. The goodness of the Dunyaa and Aakhirah comes from good manners. Their goodness is deprived by the lack of Adab.”
[مدارج السالكين ٣٦٨/٢]