Musnad Ahmad 32:1178

Mu'adh ibn Mu'adh narrated to us, Humayd al-Tawil and Ibn Abi 'Adi narrated to us, from Humayd al-Tawil, from Anas ibn Malik, that the Prophet (peace and blessings of Allah be upon him) entered the mosque and saw a rope stretched between two pillars—Ibn Abi 'Adi said: in the mosque—so he asked about it. They said: "So-and-so is praying, and when she is overcome [by sleep], she holds onto it." He said: "Let her pray as long as she is alert, and when she is overcome [by sleep], let her sleep."