Chapter 29: Medicine (Kitab Al-Tibb)
Judamat al-Asadiyyah said that she heard the Messenger of Allah (May peace be upon him) Say: I intended to prohibit suckling during pregnancy (ghailah), but I considered the Greeks and the Persians and saw that they practiced it, without any injury being caused to their children thereby. Malik said: Ghailah means that a man has intercourse with a women while she is suckling a child.
Sunan Abi Dawud 3882