When any of you marries a woman or purchases a maid-servant then let him say :
191. Allaahumma 'innee 'as'aluka khayrahaa wa khayra majabaltahaa 'alayhi wa 'a'oothu bika min sharrihaa wa sharri maajabaltahaa 'alayhi.
O Allah, I ask You for the goodness of her and the goodness upon which You have created her, and I seek refuge in You from the evil of her and from the evil upon which You have created her.
If you purchase a camel then take hold of the top of its hump and say the same.
Reference: Abu Dawud 2/248 and Ibn Majah 1/617. See also Al-Albani, Sahih Ibn Majah 1/324.