Musnad Ahmad 1153:2290

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Hisham ibn 'Urwah narrated to me, from his father, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, she said: Al-Hawla' bint Tuwayt passed by the Messenger of Allah, may Allah bless him and grant him peace, and it was said to him: "O Messenger of Allah, she prays a great deal of prayer at night, and when sleep overcomes her, she ties herself with a rope and hangs from it." He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Let her pray as much as she has the strength for, and when she becomes drowsy, let her sleep."