TIREDNESS DURING THE VOLUNTARY PRAYER H H 147 "The Prophet, praise and peace be upon him, went into the Mosque and noticed a rope stretched between two columns. He inquired: 'What is that rope for?' He was told: 'This is Zainab's, (wife of the Prophet, Mother of Believers, may Allah be pleased with her) rope. When she feels tired during her voluntary prayer she holds on to it for support.' The Prophet, praise and peace be upon him, told them: 'Undo it. You should pray so long as you are not tired. When you feel tired you should go to sleep.'" R 147 Bukhari and Muslim with a chain up to Anas who related this.
146- وعن أنس رضي الله عنه ، قَالَ : دَخَلَ النَّبيُّ صلى الله عليه وآله وسلم المَسْجِدَ فَإِذَا حَبْلٌ مَمْدُودٌ بَيْنَ السَّارِيَتَيْنِ ، فَقَالَ : "مَا هَذَا الحَبْلُ ؟" قالُوا : هَذَا حَبْلٌ لِزَيْنَبَ ، فَإِذَا فَتَرَتْ تَعَلَّقَتْ بِهِ . فَقَالَ النَّبيُّ صلى الله عليه وآله وسلم : "حُلُّوهُ ، لِيُصلِّ أَحَدُكُمْ نَشَاطَهُ فَإِذَا فَتَرَ فَلْيَرْقُدْ" مُتَّفَقٌ عَلَيهِ .