Sahih Bukhari Volume 2, Book 19, Hadith 1151

حَدَّثَنَا أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ عَبْدِ الْعَزِيزِ بْنِ صُهَيْبٍ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ قَالَ دَخَلَ النَّبِيُّ صلى الله عليه وسلم فَإِذَا حَبْلٌ مَمْدُودٌ بَيْنَ السَّارِيَتَيْنِ فَقَالَ ‏"‏ مَا هَذَا الْحَبْلُ ‏"‏‏.‏ قَالُوا هَذَا حَبْلٌ لِزَيْنَبَ فَإِذَا فَتَرَتْ تَعَلَّقَتْ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ لاَ، حُلُّوهُ، لِيُصَلِّ أَحَدُكُمْ نَشَاطَهُ، فَإِذَا فَتَرَ فَلْيَقْعُدْ ‏"

Narrated 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."