Musnad Abu Ya'la al-Mawsili 129:69

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Qudamah ibn Shihab narrated to us, Humayd narrated to us, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, entered a house or a place and saw a rope stretched out. He said, "What is this?" They said, "So-and-so is praying, and when she becomes exhausted, she holds onto it." He said, "Let her pray, and when she becomes exhausted, let her sleep or sit down."