Mishkat al-Masabih 13 Marriage كتاب النكاح 114

وَعَنْ أَسْمَاءَ بِنْتِ يَزِيدَ قَالَتْ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «لَا تَقْتُلُوا أَوْلَادَكُمْ سِرًّا فَإِنَّ الْغَيْلَ يُدْرِكُ الْفَارِسَ فيدعثره عَن فرسه» . رَوَاهُ أَبُو دَاوُد

Asma’ daughter of Yazid said she heard God’s Messenger say, “Do not kill your children secretly, for the milk with which a child is suckled while his mother is pregnant overtakes the horseman and throws him from his horse.”* * It is here said that a child which is suckled while his mother is pregnant will suffer from it in later life, the tradition speaking specifically of his being thrown from his horse and killed. Abu Dawud transmitted it.