Musannaf Ibn Abi Shayba 3:1025

Hushaym narrated to us, he said: Humayd informed us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) entered one day and found a rope stretched out. He said: "What is this?" It was said: "So-and-so is praying, O Messenger of Allah, and when she becomes exhausted, she rests upon this rope." He said: "Let her pray as long as she is energetic, and when she becomes exhausted, let her sleep."