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 you marry a woman or buy a maidservant, you should say:(O Allah, I ask You for the goodness within her and the goodness that you have made her inclined towards, and I take refuge with You from the evil within her and the evil that you have made her inclined towards.))(and if you buy a camel, then you should take hold of it’s hump and say likewise.)
Abu Dawud 2/248 and Ibn Majah 1/617. See also Al-Albani, Sahih Ibn Majah 1/324.