Musnad Ahmad 32:46

Isma'il narrated to us, 'Abd al-'Aziz ibn Suhayb narrated to us, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) entered the mosque and there was a rope stretched between two pillars. He said: "What is this?" They said: "It is for Zaynab; she prays, and when she becomes lazy or weary, she holds onto it." He said: "Untie it." Then he said: "Let one of you pray while he is energetic, and when he becomes lazy or weary, let him sit down."