Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) forbade 'ightiyal', and then said: "If it harmed anyone, it would certainly have harmed Persia and Rome."
Ibn Bukayr said: Ightiyal means that a man has intercourse with his wife while she is breastfeeding a child.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11341