Musnad Ahmad 1304:3

Al-Fadl ibn Dukayn narrated to us, Ibn Abi Ghunya narrated to us, from Muhammad ibn Muhajir, from his father, from Asma bint Yazid, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Do not kill your children secretly, for the killing of the *ghayl* (nursing a child while pregnant) overtakes the horseman and throws him off the back of his horse."