(46) باب مَا يَجْمَعُ صِفَةَ الصَّلاَةِ وَمَا يُفْتَتَحُ بِهِ وَيُخْتَمُ بِهِ وَصِفَةَ الرُّكُوعِ وَالاِعْتِدَالِ مِنْهُ وَالسُّجُودِ وَالاِعْتِدَالِ مِنْهُ وَالتَّشَهُّدِ بَعْدَ كُلِّ رَكْعَتَيْنِ مِنَ الرُّبَاعِيَّةِ وَصِفَةَ الْجُلُوسِ بَيْنَ السَّجْدَتَيْنِ وَفِي التَّشَهُّدِ الأَوَّلِ
This hadith has been narrated by Ja'far b. Rabi' with the same chain of transmitters. And in the narration transmitted by 'Amr b. al-Harith (the words are):
" When the Messenger of Allah (rtiay peace be upon him) prostrated, he spread out his arms so that the whiteness of his armpits was visible." And in the narration transmitted by al-Laith (the words are:" When the Messenger of Allah (ﷺ) prostrated. he spread his hands from the armpits so that I saw their whiteness."
Reference: Sahih Muslim 495 b
In-book reference: Book 4, Hadith 268
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1001