The Prophet (peace and blessings be upon him) said:
"If this (breastfeeding during pregnancy) were harmful, it would certainly have harmed Persia and Rome."
Hadith Referenceصحیح الجامع الصغیر / حرف اللام / 5285
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[م] عن أسامة بن زيد. المشكاة ٣١٨٨، م ٤/١٦٢.