Mishkat al-Masabih 4 Prayer كتاب الصلاة 315

وَعَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ بن بُحَيْنَة قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سَجَدَ فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاض إبطَيْهِ

‘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.)