سنن ابن ماجه

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 (ﷺ) 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."