Prostration and its Excellence - Section 1 Mishkat al-Masabih 891
Arabic hadith text
وَعَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ بن بُحَيْنَة قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سَجَدَ فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاض إبطَيْهِ
Translation
‘Abdallah b. Malik Ibn Buhaina* said that when the Prophet prostrated himself he spread out his arms so that the whiteness under his armpits was visible. * Buhaina was ‘Abdallah’s mother.(Bukhari and Muslim.)