Musnad Ahmad 1153:355

Hassan ibn Musa narrated to us, he said: Ibn Lahi'ah narrated to us, Yazid ibn Abi Habib narrated to us, from Musa ibn Sa'id ibn Zayd ibn Thabit, from Khubayb ibn Abdullah ibn al-Zubayr, from Aisha, the wife of the Prophet, may God bless him and grant him peace, she said: The menses came upon me during the night while I was at the side of the Messenger of God, may God bless him and grant him peace, so I moved away. He said: "What is the matter with you? Have you become menstruating?" She said: "No, but I have begun my menses." He said: "Then fasten your lower garment upon yourself, then return."