When the Prophet (may Allah's peace and blessings be upon him) prayed, he used to spread out his hands until the whiteness of his armpits would be visible

When prostrating, the Prophet (may Allah's peace and blessings be upon him) used to spread out his hands during Sujūd (prostration), keeping each hand away from its respective side like the two wings, until the skin color of his armpits would become visible. This is a form of exaggeration in distancing the arms and keeping them far from his sides.