Sahih Bukhari
Good Manners and Form (Al-Adab)
Volume: 008 / Book Number: 073 / Hadith Number: 58
Narrated Anas bin Malik:
The Prophet was not one who would abuse (others) or say obscene words, or curse (others), and if he wanted to admonish anyone of us, he used to say: "What is wrong with him, his forehead be dusted!"