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.
Abu Dawud 2/248 and Ibn Majah 1/617. See also Al-Albani, Sahih Ibn Majah 1/324.