The Messenger of Allah ﷺ, when he prayed, would spread his arms apart so that the whiteness of his armpits became visible.In another wording, when he prostrated, he would spread his arms apart.
أن رسول الله ﷺ كان إذا صلى فرج بين يديه، حتى يبدو بياض إبطيه. وفي لفظ: كان إذا سجد فرج بين يديه.
Bukhari narrated it in the Book of Virtues (3564), and Muslim narrated it in the Book of Prayer (49:5235), both through the route of Bakr bin Mudar, from Jafar bin Rabia, from Araj, from Abdullah bin Malik bin Buhayna, who said, and then he mentioned it.