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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.)
Source: Abu Dawud 2/248, Ibn Majah 1/617.