It is narrated from Sayyiduna Abdullah bin Malik bin Buhaynah (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) prayed, he kept his arms spread apart, so that even the whiteness of his armpits could be seen.
In the narration of Abu Salih al-Juhani: when the Messenger of Allah (peace and blessings be upon him) prostrated, he kept his upper arms away from his sides, so that the whiteness of his armpits could be seen.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2703