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