Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(127)
Chapter: To stand Straight with calmness on raising the head from bowing
(127)
باب الاِطْمَأْنِينَةِ حِينَ يَرْفَعُ رَأْسَهُ مِنَ الرُّكُوعِ
Sahih al-Bukhari 800

Narrated Thabit:

Anas used to demonstrate to us the prayer of the Prophet (ﷺ) and while demonstrating, he used to raise his head from bowing and stand so long that we would say that he had forgotten (the prostration).

حَدَّثَنَا أَبُو الْوَلِيدِ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ ثَابِتٍ، قَالَ كَانَ أَنَسٌ يَنْعَتُ لَنَا صَلاَةَ النَّبِيِّ صلى الله عليه وسلم فَكَانَ يُصَلِّي وَإِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ قَامَ حَتَّى نَقُولَ قَدْ نَسِيَ‏.‏
Reference : Sahih al-Bukhari 800
In-book reference : Book 10, Hadith 195
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 765
  (deprecated numbering scheme)