Urwah bin al-Zubayr (may Allah have mercy on him) said: I heard Mughirah bin Shu'bah (may Allah be pleased with him) saying that Umar (may Allah be pleased with him) asked them about imlas al-mar'ah. Mughirah (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) decided in it a ghurrah of a male or female slave." Umar (may Allah be pleased with him) said: "Who will confirm what you say?" So Muhammad bin Maslamah (may Allah be pleased with him) confirmed it for him.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18194