It is narrated from Sayyidah Judamah bint Wahb al-Asadiyyah (may Allah be pleased with her) that she heard the Messenger of Allah (peace and blessings be upon him) say: "I intended to forbid ghilah (a nursing woman becoming pregnant), until I remembered that the Persians and the Romans do that and it does not harm their children."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20904