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."
English reference: Vol. 3, Book 9, Hadith 2012
Arabic reference: Book 9, Hadith 2090