She narrated to her that the Messenger of Allah ﷺ said, "I had nearly resolved to forbid intercourse with a nursing wife, until I recalled that the Persians and the Romans practice it, and it does not harm their children."
حدثنا عبد الرحمن بن مهدي عن مالك عن أبي الأسود عن عروة عن عائشة أن
حدثتها أن رسول الله ﷺ قال لقد هممت أن أنهى عن الغيلة حتى ذكرت أن فارس والروم يصنعونه فلا يضر أولادهم
Its isnad is sahih. Muslim reported it, 2/1066, no. 1442, and Nasai, 6/106, no. 3326, and Ibn Majah, 1/648, no. 2011, and Darimi, 2/147, no. 2217, in the chapter on marriage, and Abu Dawud, 4/9, no. 3882, in the chapter on medicine, and Malik, 2/707, no. 16, in the chapter on suckling.Ghayl means that a woman becomes pregnant while she is nursing. Many have explained it as a man having intercourse with his wife while she is nursing, as though they took that as supporting evidence for it.