Sunan Ibn Majah, Volume 3, Book 9, Hadith 2012 (DarusSalam) ⓘ

It was narrated from Muhajir bin Abu Muslim, from Asma' bint Yazid bin Sakary who was his freed slave womary that: she heard the Messenger of Allah say: "Do not kill your children secretly, for by the One in Whose Hand is my soul, intercourse with a breastfeeding woman catches up with people when they are riding their horses (in battle) and wrestles them to the ground."

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا يَحْيَى بْنُ حَمْزَةَ، عَنْ عَمْرِو بْنِ مُهَاجِرٍ، أَنَّهُ سَمِعَ أَبَاهُ الْمُهَاجِرَ بْنَ أَبِي مُسْلِمٍ، يُحَدِّثُ عَنْ أَسْمَاءَ بِنْتِ يَزِيدَ بْنِ السَّكَنِ، وَكَانَتْ، مَوْلاَتَهُ أَنَّهَا سَمِعَتْ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ يَقُولُ ‏ "‏ لاَ تَقْتُلُوا أَوْلاَدَكُمْ سِرًّا فَوَالَّذِي نَفْسِي بِيَدِهِ إِنَّ الْغَيْلَ لَيُدْرِكُ الْفَارِسَ عَلَى ظَهْرِ فَرَسِهِ حَتَّى يَصْرَعَهُ ‏" ‏ ‏.‏