It is narrated from Sayyidah Asma bint Yazid (may Allah be pleased with her), and she was the mawlah (freed client) of (the narrator Muhajir), that she heard the Messenger of Allah (peace and blessings be upon him) say: "Do not kill your children secretly. By the One in whose hand is my soul, ghilah (that is, a nursing woman becoming pregnant) overtakes the rider on the back of his horse until it throws him down."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20833