Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(140)
Chapter: To sit for a while between the two prostrations
(140)
باب الْمُكْثِ بَيْنَ السَّجْدَتَيْنِ
Sahih al-Bukhari 821

Narrated Thabit:

Anas said, "I will leave no stone unturned in making you offer the prayer as I have seen the Prophet (ﷺ) making us offer it." Anas used to do a thing which I have not seen you doing. He used to stand after the bowing for such a long time that one would think that he had forgotten (the prostrations) and he used to sit in-between the prostrations so long that one would think that he had forgotten the second prostration.

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ زَيْدٍ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ إِنِّي لاَ آلُو أَنْ أُصَلِّيَ بِكُمْ كَمَا رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يُصَلِّي بِنَا‏.‏ قَالَ ثَابِتٌ كَانَ أَنَسٌ يَصْنَعُ شَيْئًا لَمْ أَرَكُمْ تَصْنَعُونَهُ، كَانَ إِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ قَامَ حَتَّى يَقُولَ الْقَائِلُ قَدْ نَسِيَ‏.‏ وَبَيْنَ السَّجْدَتَيْنِ حَتَّى يَقُولَ الْقَائِلُ قَدْ نَسِيَ‏.‏
Reference : Sahih al-Bukhari 821
In-book reference : Book 10, Hadith 215
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 784
  (deprecated numbering scheme)