Reference : Hisn al-Muslim 191

إِذَا تَزَوَّجَ أَحَدُكُمُ امْرَأَةً، أَوْ إِذَا اشْتَرَى خَادِماً فَلْيَقُلْ: 
(اللَّهُمَّ إِنِّي أَسْأَلُكَ خَيْرَهَا، 
وَخَيْرَ مَا جَبَلْتَهَا عَلَيْهِ، 
وَأَعُوذُ بِكَ مِنْ شَرِّهَا، 
وَشَرِّ مَا جَبَلْتَهَا عَلَيْهِ)، 
وَإِذَا اشْتَرَى بَعِيراً فَلْيَأْخُذْ بِذِرْوَةِ سَنَامِهِ وَلْيَقُلْ مِثْلَ ذَلِكَ.
When any of you marries a woman or purchases a maid-servant then let him say:
Allāhumma innī as'aluka khayrahā 
wa khayra mā jabaltahā `alayh,
wa a`ūdhu bika min sharrihā 
wa sharri mā jabaltahā `alayh.


When any of you marries a woman or purchases a maid-servant then let him say :

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.

Chapter: On the wedding night or when buying an animal