From Sahih Muslim (Hadith 1764):
Abu Sa'id (al Khudri) reported Allah's Apostle (ﷺ) as saying:Observe Witr prayer before it is morning. Abu Sa'id reported that they (the Prophet's Companions) asked the Messenger of Allah (ﷺ) about Witr (prayer). (In reply to their inquiry) he said: Observe Witr prayer before it is morning
Reference: Muslim 1764