Abu Dawud Book 2, Hadith Number 1432.

Narated By ‘Abd Allah b. Abu Qais : I asked ‘Aisha about the witr observed by the Apostle of Allah (PBUH). She replied: Sometimes he observed the witr prayer in the early hours of the night, sometimes he observed it at the end of it. I asked: How did he recite the Quran? Did he recite the Quran quietly or loudly? She replied: He did it in any way. Sometimes he recited quietly and sometimes loudly; sometimes he took a bath and then slept and sometimes he performed ablution and then slept.

Abu Dawud said: The narrators other than Qutaibah said: This refers to his bath due to sexual defilement.