'Amr b. Shu'aib on his father's authority said that his grandfather (Abdullah ibn Amr ibn al-'As) reported the Prophet ﷺ said:

If one of you marries a woman or buys a slave, he should say: "O Allah, I ask You for the good in her, and in the disposition You have given her; I take refuge in You from the evil in her, and in the disposition You have given her." When he buys a camel, he should take hold of the top of its hump and say the same kind of thing.

Abu Dawud said: Abu Sa'id added the following words in his version: He should then tale hold of her forelock and pray for blessing in the case of a woman or a slave.

حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، وَعَبْدُ اللَّهِ بْنُ سَعِيدٍ، قَالاَ حَدَّثَنَا أَبُو خَالِدٍ، - يَعْنِي سُلَيْمَانَ بْنَ حَيَّانَ - عَنِ ابْنِ عَجْلاَنَ، عَنْ عَمْرِو بْنِ شُعَيْبٍ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، عَنِ النَّبِيِّ ﷺ قَالَ " إِذَا تَزَوَّجَ أَحَدُكُمُ امْرَأَةً أَوِ اشْتَرَى خَادِمًا فَلْيَقُلِ اللَّهُمَّ إِنِّي أَسْأَلُكَ خَيْرَهَا وَخَيْرَ مَا جَبَلْتَهَا عَلَيْهِ وَأَعُوذُ بِكَ مِنْ شَرِّهَا وَمِنْ شَرِّ مَا جَبَلْتَهَا عَلَيْهِ وَإِذَا اشْتَرَى بَعِيرًا فَلْيَأْخُذْ بِذِرْوَةِ سَنَامِهِ وَلْيَقُلْ مِثْلَ ذَلِكَ " . قَالَ أَبُو دَاوُدَ زَادَ أَبُو سَعِيدٍ " ثُمَّ لْيَأْخُذْ بِنَاصِيَتِهَا وَلْيَدْعُ بِالْبَرَكَةِ " . فِي الْمَرْأَةِ وَالْخَادِمِ .