During Prostrations one should show his armpits and separate his forearms from his body

Narrated 'Abdullah bin Malik:

Ibn Buhaina, "When the Prophet prayed, he used to separate his arms from his body so widely that the whiteness of his armpits was visible."

حَدَّثَنَا إِسْحَاقُ بْنُ نَصْرٍ، قَالَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنِ الأَعْمَشِ، عَنْ مُسْلِمٍ، عَنْ مَسْرُوقٍ، عَنِ الْمُغِيرَةِ بْنِ شُعْبَةَ، قَالَ وَضَّأْتُ النَّبِيَّ ﷺ فَمَسَحَ عَلَى خُفَّيْهِ وَصَلَّى.