Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

from his father that his grandfather told that the Messenger of Allah (saw) said: "When anyone of you buys a slave woman let him say: 'Allahumma inni as'aluka khairaha wa khaira ma jabaltaha alaihi, wa a'udhu bika min sharriha wa sharri ma jabaltaha alaihi (O Allah, I ask You for the goodness within her and the goodness that You have made her inclined towards, and I seek refuge with You from the evil within her and the evil that You have made her inclined towards).' And he should pray for blessing. And if anyone of you buys a camel then he should take hold of its hump and pray for. blessing and say similar words."