Musnad Ahmad 32:972

'Abd al-Rahman narrated to us, Hammad ibn Salamah narrated to us, from Thabit, from 'Abd al-Rahman ibn Abi Layla, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw a rope stretched between two pillars, so he said: "Whose is this?" They said: "It belongs to Hamnah bint Jahsh; she prays, and when she becomes weary, she holds onto it." He said: "Let her pray as much as she is able, and when she becomes weary, let her sit."