Sahih Bukhari Volume 002, Book 021, Hadith Number 251A.
Narated By Anas bin Malik : Once the Prophet (p.b.u.h) entered the Mosque and saw a rope hanging in between its two pillars. He said, "What is this rope?" The people said, "This rope is for Zainab who, when she feels tired, holds it (to keep standing for the prayer.)" The Prophet said, "Don't use it. Remove the rope. You should pray as long as you feel active, and when you get tired, sit down."