Sahih Muslim Book - 4, Hadith - 1060

حَدَّثَنَا خَلَفُ بْنُ هِشَامٍ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ عَنْ ثَابِتٍ عَنْ أَنَسٍ قَالَ إِنِّى لاَ آلُو أَنْ أُصَلِّىَ بِكُمْ كَمَا رَأَيْتُ رَسُولَ اللَّهِ -صلى الله عليه وسلم- يُصَلِّى بِنَا. قَالَ فَكَانَ أَنَسٌ يَصْنَعُ شَيْئًا لاَ أَرَاكُمْ تَصْنَعُونَهُ كَانَ إِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ انْتَصَبَ قَائِمًا حَتَّى يَقُولَ الْقَائِلُ قَدْ نَسِىَ. وَإِذَا رَفَعَ رَأْسَهُ مِنَ السَّجْدَةِ مَكَثَ حَتَّى يَقُولَ الْقَائِلُ قَدْ نَسِىَ.

It was narrated from Thabit, that Anas said: "I try my best to lead you in prayer as I saw the Messenger of Allah (s.a.w) lead us in prayer." He said: "Anas used to do something that I have not seen you do. When he raised his head from bowing, he would stand so long that one would think that he had forgotten, and when he raised his head from prostrating, he would remain so long that one would think that he had forgotten."