Sayyiduna Ibn Abbas (may Allah be pleased with them both) says: I came to the Prophet (peace and blessings be upon him) from behind him. I saw the whiteness of his armpits. He (peace and blessings be upon him) had kept both his arms and both his hands apart.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2708