It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) performed prostration, the whiteness of his blessed armpits could be seen (i.e., his arms were so separated).
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 2073
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف، شعبة مولى ابن عباس سيء الحفظ.