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." (FR)
English reference (FR): Vol. 1, Book 1, Hadith 654 (FR)
Arabic reference (FR): Book 1, Hadith 698 (FR)