(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
'Abdullah b. Malik ibn Bujainah reported:
When the Prophet (ﷺ) prostrated, lie spread out his arms so that the whiteness of his armpits was visible.
Reference: Sahih Muslim 495 a
In-book reference: Book 4, Hadith 267
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1000