It was narrated from 'Aishah that:
The Prophet entered upon her, and a freed slave girl of hers concealed herself. The Prophet asked: "Have her periods begun?" She said: "Yes." He tore a piece of his turban and said: "Cover your head with this." (IT)
English reference (IT): Vol. 1, Book 1, Hadith 654 (IT)
Arabic reference (IT): Book 1, Hadith 698 (IT)