Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1714
Anas reported that the Messenger of Allah (may peace be upon him) entered the mosque (and he found) a rope tied between the two pillars; so he said: What is this? They said: It is for Zainab. She prays and when she slackens or feels tired she holds it. Upon this he (the Holy Prophet) said: Untie it. Let one pray as long as one feels fresh but when one slackens or becomes tired one must stop it. (And in the hadith transmitted by Zuhair it is:" He should sit down." )