Musnad Abu Ya'la al-Mawsili 129:114

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Thabit, from Abd al-Rahman ibn Abi Layla, and Humayd, from Anas ibn Malik: That the Messenger of Allah (peace be upon him) saw a rope stretched between two pillars in the mosque. The Messenger of Allah (peace be upon him) said: "What is this rope?" It was said: "O Messenger of Allah, it belongs to Hamnah bint Jahsh; she prays, and when she becomes exhausted, she hangs onto it." The Messenger of Allah (peace be upon him) said: "Let her pray, and when she becomes exhausted, let her sit down."