The Book of Prayer - Travellers
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ الأَعْلَى بْنُ عَبْدِ الأَعْلَى، عَنْ مَعْمَرٍ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " أَوْتِرُوا قَبْلَ أَنْ تُصْبِحُوا " .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
From Sahih Muslim, whose reports the scholars accept as authentic.