Anas ibn Malik reported: The Prophet, peace and blessings be upon him, entered the mosque and saw a rope between two pillars. He asked, “What is this rope?” They said, “This rope is for Zainab who uses it when she becomes tired during prayer.” The Prophet said, “Do not use it. You should pray when you have energy, but if you feel tired you should sit down.”

Source: Sahih Bukhari 1099

Grade: Sahih (authentic) according to Al-Bukhari

عَنْ أَنَسِ بْنِ مَالِكٍ رَضِيَ اللَّهُ عَنْهُ قَالَ دَخَلَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَإِذَا حَبْلٌ مَمْدُودٌ بَيْنَ السَّارِيَتَيْنِ فَقَالَ مَا هَذَا الْحَبْلُ قَالُوا هَذَا حَبْلٌ لِزَيْنَبَ فَإِذَا فَتَرَتْ تَعَلَّقَتْ فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا حُلُّوهُ لِيُصَلِّ أَحَدُكُمْ نَشَاطَهُ فَإِذَا فَتَرَ فَلْيَقْعُدْ

1099 صحيح البخاري كتاب التَّهَجُّد بَاب مَا يُكْرَهُ مِنْ التَّشْدِيدِ فِي الْعِبَادَةِ