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.  

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  

Ibn ‘Abbas reported the Prophet ﷺ as saying “If anyone who means to have intercourse with his wife says “In the name of Allaah, O Allaah, keep us away from the devil and keep the devil away from what You hast provided us.” It will be ordained that no devil will ever harm the child born to them.  

قَالَ النَّبِيُّ ﷺ لَوْ أَنَّ أَحَدَكُمْ إِذَا أَرَادَ أَنْ يَأْتِيَ أَهْلَهُ قَالَ بِسْمِ اللَّهِ اللَّهُمَّ جَنِّبْنَا الشَّيْطَانَ وَجَنِّبِ الشَّيْطَانَ مَا رَزَقْتَنَا ثُمَّ قُدِّرَ أَنْ يَكُونَ بَيْنَهُمَا وَلَدٌ فِي ذَلِكَ لَمْ يَضُرَّهُ شَيْطَانٌ أَبَدًا  

The Prophet ﷺ said: He who has intercourse with his wife through her anus is accursed.  

قَالَ رَسُولُ اللَّهِ ﷺ مَلْعُونٌ مَنْ أَتَى امْرَأَتَهُ فِي دُبُرِهَا  

Muhammad bin Al Munkadir said I heard Jabir say The Jews used to say “When a man has intercourse with his wife through the vagina, but being on her back the child will have a squint, so the verse came down. Your wives are a tilth to you, so come to your tilth however you will.”  

سَمِعْتُ جَابِرًا يَقُولُ إِنَّ الْيَهُودَ يَقُولُونَ إِذَا جَامَعَ الرَّجُلُ أَهْلَهُ فِي فَرْجِهَا مِنْ وَرَائِهَا كَانَ وَلَدُهُ أَحْوَلَ فَأَنْزَلَ اللَّهُ سُبْحَانَهُ وَتَعَالَى { نِسَاؤُكُمْ حَرْثٌ لَكُمْ فَأْتُوا حَرْثَكُمْ أَنَّى شِئْتُمْ }  

Ibn Umar misunderstood (the Qur'anic verse, "So come to your tilth however you will")--may Allah forgive him. The fact is that this clan of the Ansar, who were idolaters, lived in the company of the Jews who were the people of the Book. They (the Ansar) accepted their superiority over themselves in respect of knowledge, and they followed most of their actions. The people of the Book (i.e. the Jews) used to have intercourse with their women on one side alone (i.e. lying on their backs). This was the most concealing position for (the vagina of) the women. This clan of the Ansar adopted this practice from them. But this tribe of the Quraysh used to uncover their women completely, and seek pleasure with them from in front and behind and laying them on their backs. When the muhajirun (the immigrants) came to Medina, a man married a woman of the Ansar. He began to do the same kind of action with her, but she disliked it, and said to him: We were approached on one side (i.e. lying on the back); do it so, otherwise keep away from me. This matter of theirs spread widely, and it reached the Messenger of Allah ﷺ. So Allah, the Exalted, sent down the Qur'anic verse: "Your wives are a tilth to you, so come to your tilth however you will," i.e. from in front, from behind or lying on the back. But this verse meant the place of the delivery of the child, i.e. the vagina.  

إِنَّ ابْنَ عُمَرَ وَاللَّهُ يَغْفِرُ لَهُ أَوْهَمَ إِنَّمَا كَانَ هَذَا الْحَىُّ مِنَ الأَنْصَارِ وَهُمْ أَهْلُ وَثَنٍ مَعَ هَذَا الْحَىِّ مِنْ يَهُودَ وَهُمْ أَهْلُ كِتَابٍ وَكَانُوا يَرَوْنَ لَهُمْ فَضْلاً عَلَيْهِمْ فِي الْعِلْمِ فَكَانُوا يَقْتَدُونَ بِكَثِيرٍ مِنْ فِعْلِهِمْ وَكَانَ مِنْ أَمْرِ أَهْلِ الْكِتَابِ أَنْ لاَ يَأْتُوا النِّسَاءَ إِلاَّ عَلَى حَرْفٍ وَذَلِكَ أَسْتَرُ مَا تَكُونُ الْمَرْأَةُ فَكَانَ هَذَا الْحَىُّ مِنَ الأَنْصَارِ قَدْ أَخَذُوا بِذَلِكَ مِنْ فِعْلِهِمْ وَكَانَ هَذَا الْحَىُّ مِنْ قُرَيْشٍ يَشْرَحُونَ النِّسَاءَ شَرْحًا مُنْكَرًا وَيَتَلَذَّذُونَ مِنْهُنَّ مُقْبِلاَتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ فَلَمَّا قَدِمَ الْمُهَاجِرُونَ الْمَدِينَةَ تَزَوَّجَ رَجُلٌ مِنْهُمُ امْرَأَةً مِنَ الأَنْصَارِ فَذَهَبَ يَصْنَعُ بِهَا ذَلِكَ فَأَنْكَرَتْهُ عَلَيْهِ وَقَالَتْ إِنَّمَا كُنَّا نُؤْتَى عَلَى حَرْفٍ فَاصْنَعْ ذَلِكَ وَإِلاَّ فَاجْتَنِبْنِي حَتَّى شَرِيَ أَمْرُهُمَا فَبَلَغَ ذَلِكَ رَسُولَ اللَّهِ ﷺ فَأَنْزَلَ اللَّهُ ﷻ { نِسَاؤُكُمْ حَرْثٌ لَكُمْ فَأْتُوا حَرْثَكُمْ أَنَّى شِئْتُمْ } أَىْ مُقْبِلاَتٍ وَمُدْبِرَاتٍ وَمُسْتَلْقِيَاتٍ يَعْنِي بِذَلِكَ مَوْضِعَ الْوَلَدِ