When the Messenger of Allah ﷺ prostrated, he kept his hands so much apart from each other that when it was seen from behind the armpits became visible. Waki' said: That is their whiteness.
كَانَ رَسُولُ اللَّهِ ﷺ إِذَا سَجَدَ جَافَى حَتَّى يَرَى مَنْ خَلْفَهُ وَضَحَ إِبْطَيْهِ قَالَ وَكِيعٌ يَعْنِي بَيَاضَهُمَا