Marriage (Kitab Al-Nikah)
Dawud :: Book 11 : Hadith 2155
Narrated Abdullah ibn Amr ibn al-'As:
The Prophet (peace_be_upon_him) said: If one of you marries a woman or buys a slave, he should say: "O Allah, I ask Thee for the good in her, and in the disposition Thou hast given her; I take refuge in Thee from the evil in her, and in the disposition Thou hast given her." When he buys a camel, he should take hold of the top of its hump and say the same kind of thing.