عَنْ أَنَسِ بْنِ مَالِكٍ رَضِيَ اللَّهُ عَنْهُ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ إِذَا سَجَدَ رُؤِيَ أَوْ رَأَيْتُ بَيَاضَ إِبْطَيْهِ
Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that when the Messenger of Allah (peace and blessings be upon him) performed prostration, the whiteness of his armpits could be seen.