Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When Sayyiduna Musa (peace be upon him) entered the water, he would not take off his garment until his private parts were concealed in the water."
Ahmad recorded it, and its narrators have been declared trustworthy, except that there is disagreement about using the narrator Ali bin Zayd as evidence.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1458