Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) forbade ghayl, then he said: It does not harm the Persians and the Romans. (The narrator says:) It means that a man has relations with his wife while she is breastfeeding.
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains Layth bin Hammad, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب النكاح / 7590