When the Prophet ﷺ prayed, he would hold his arms out from his sides until the whiteness of his armpits became visible.
أن النبي ﷺ كان إذا صلى فرج بين يديه حتى يبدو بياض إبطيه.
Bukhari related it in the Book of Prayer (390), and Muslim in the Book of Prayer (495), both through the route of Bakr bin Mudar, from Jafar bin Rabia, from Araj, from Abdullah bin Malik bin Buhaynah, and he narrated it as mentioned.In a version reported by Muslim, "When he prostrated, he would spread his arms out in his prostration until the whiteness of his armpits could be seen."Nawawi said, "Tafrij, tajnih, and takhwiyah all carry the same meaning, and their meaning in every case is that he would keep his elbows and upper arms away from his sides."