The Messenger of Allah ﷺ entered the mosque and saw a rope stretched between two pillars.He asked, "What is this?"They replied, "It belongs to Zaynab. She prays, and when she grows tired or weak, she holds onto it for support."He said, "Untie it." Then he said, "Let one of you pray as long as he feels energetic, and when he grows tired or weak, let him sit down."
حدثنا إسماعيل حدثنا عبد العزيز بن صهيب
دخل رسول الله ﷺ المسجد وحبل ممدود بين ساريتين فقال ما هذا قالوا لزينب تصلي فإذا كسلت أو فترت أمسكت به فقال حلوه ثم قال ليصل أحدكم نشاطه فإذا كسل أو فتر فليقعد
Its isnad is sahih. Bukhari related it 2/67 in Prayer, chapter on the dislike of exerting oneself excessively, Muslim 1/542 no. 784 in Travelers, chapter on the command for one who feels drowsy, Abu Dawud 2/34 no. 1312, and Nasai 3/219 no. 1643.