Mishkat al-Masabih #2340

'Amr b. Shu'aib, on his father’s authority, said his grandfather told how the Prophet said that when anyone of them married a woman, or bought a servant he should say, “O God, I ask Thee for the good in her and in the disposition Thou hast given her, and I seek 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. In a version regarding a woman and a servant it says he should take hold of her forelock and make supplication for a blessing. Abu Dawud and Ibn Majah transmitted it.