Authentic
graded by Al-Albani
Mishkat al-Masabih 1244
Narrator:Anas ibn Mālik
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.)
Cited in Mishkat al-Masabih (No. 1244).