Authentic Supplication Number 240.

It is reported that the Prophet  said: "If one of you marries a woman, or buys a servant, then let him say: 'Allahumma 'innee 'as'aluka khayraha, wa khayra ma jabaltaha `alayhi, wa 'a`outhu bika min sharriha wa sharri ma jabaltaha `alayhi (250), and if he purchases a camel, then let him hold it by the tip of its hump , and let him say the same."

Footnotes.
(250) O' my ilah I ask You her goodness, and the goodness of the inborn dispositions which You have given her, and I solicit Your protection from her evil, and the evil dispositions which You have given her.

Reporters.
Reported by Abu Dawood. Al-Albani said that it was sound according to the conditions of Al-Bukhari, and Muslim, and that it was also reported by Muslim. Al-Albani includedd this hadeeth in The Authentic of Good Sayings as #169.