Mishkat al-Masabih 4 Prayer كتاب الصلاة 659

وَعَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: ” لِيُصَلِّ أَحَدُكُمْ نَشَاطَهُ وَإِذَا فَتَرَ فَلْيَقْعُدْ)

Anas reported God’s Messenger as saying, “One should pray as long as he is lively, but when he slackens he should stop.” (Bukhari and Muslim.)