Al-A'mash (may Allah be pleased with him) said: I saw Sayyiduna Anas bin Malik (may Allah be pleased with him) praying in Makkah. When he went into prostration, he held his arms away from his sides, so that I could see the creases of his armpit.
Imam al-Bazzar recorded it; its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2770