Musnad Abu Ya'la al-Mawsili 129:126

Zuhayr narrated to us, Yazid narrated to us, Humayd narrated from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) entered the mosque and saw a rope stretched between two pillars. He said, "What is this?" They said, "It is for so-and-so; she prays, and when she becomes weary, she hangs onto it." The Messenger of Allah (may Allah bless him and grant him peace) said, "Let her pray as long as she is alert, and when she fears that she will be overcome [by sleep], let her sleep."