Musnad Ahmad 1304:25

Hammad ibn Khalid narrated to us, he said: Mu'awiyah—meaning Ibn Salih—narrated to us, from al-Muhajir, the freed slave of Asma bint Yazid al-Ansariyyah, who said: I heard Asma bint Yazid saying: I heard the Prophet, peace and blessings of Allah be upon him, saying: "Do not kill your children secretly, for by the One in Whose hand is my soul, it (the secret killing) will surely overtake the horseman and throw him from his horse." He (the narrator) said: I said: "What do you mean?" He said: "Al-ghilah (secret killing/intercourse while breastfeeding); it is when a man comes to his wife while she is breastfeeding."