Musnad Ahmad 1304:30

Abu al-Mughirah and Ali ibn Ayyash narrated to us, they said: Muhammad ibn Muhajir narrated to us, he said: My father narrated to me, from Asma bint Yazid ibn Sakan al-Ansari, she said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Do not kill your children secretly, for the *ghayl* (having intercourse with a nursing mother) overtakes the horseman and throws him from upon his horse." Ali said: Asma bint Yazid al-Ansari said: The Messenger of Allah, peace and blessings of Allah be upon him, said, and he mentioned the like of it.