´It was narrated that Abu Hurairah said:` "If I were in front of the Messenger of Allah (ﷺ) I would be able to see the whiteness of his armpits." (One of the narrators) Abu Mijlaz said: "It is as if he said that because he was praying."
Hadith Referenceسنن نسائي / كتاب التطبيق / 1108
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:إسناده حسن
1؎ : The meaning is that the Prophet (sallallahu alayhi wa sallam) would spread his hands in prostration (sajdah) to such an extent that the whiteness of his armpits would become visible.