Maliks Muwatta Book 28, Hadith Number 52.

Yahya related to me from Malik from Zayd ibn Aslam that the Messenger of Allah, may Allah bless him and grant him peace, said, “When you marry a woman or buy a slave-girl, take her by the forelock and ask for baraka. When you buy a camel, take the top of its hump, and seek refuge with Allah from Shaytan.”