Mukhtasar Muslim

Narrated Anas (may Allah be pleased with him): The Messenger of Allah (peace and blessings of Allah be upon him) entered the mosque and saw a rope hanging between two pillars. He said, "What is this?" The people said, "This is the rope of Umm al-Mu'mineen Zaynab (may Allah be pleased with her); she prays, and when she becomes sluggish or tired, she holds onto it." The Prophet (peace and blessings of Allah be upon him) said, "Untie it. Each of you should pray as long as he is energetic. When he becomes sluggish or tired, he should sit down."