Imam Bukhari narrates, Yahya bin Bukair informed us, he said: Bakr bin Mudar narrated to us, from Ja'far, from Ibn Hurmuz, from Abdullah bin Malik Ibn Buhainah (may Allah be pleased with him), he narrates that

when the Prophet ﷺ prayed, he kept both his arms spread out, until the whiteness (of the place) of his armpits became visible.

And Laith said: Ja'far bin Rabi'ah narrated to me a Hadeeth similar to it.