صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب ذكر الوتر وما فيه من السنن
Chapters on Witr and its Sunnahs
(446) باب ذكر الأخبار المنصوصة عن النبى صلى الله عليه وسلم أن الوتر ركعة
The chapter on explicit reports from the Prophet that Witr is one unit
Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) narrates that I saw the Messenger of Allah (peace and blessings be upon him), he made his mount sit down, then dismounted, and performed ten rak‘ahs and one witr. He (peace be upon him) performed two rak‘ahs at a time, then performed one rak‘ah of witr. Then he (peace be upon him) performed two rak‘ahs of Fajr (Sunnah), then led us in the morning prayer. I have mentioned this complete chapter in Kitab al-Kabir.
تخریج الحدیث: «اسناده صحيح، أخرجه ابن خزيمة فى (صحيحه) برقم: 1075، 1261، وابن حبان فى (صحيحه) برقم: 2629»