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