Sayyiduna Abu Hurairah (may Allah be pleased with him) narrated from the Messenger of Allah (peace and blessings be upon him), he (peace and blessings be upon him) said: "Whoever does not pray Witr is not one of us."
Hadith Referenceمسند اسحاق بن راهويه / كتاب التهجد و التطوع / 233
Sayyiduna Abu Hurairah (may Allah be pleased with him) narrated: The Messenger of Allah (peace and blessings be upon him) said to Bilal (may Allah be pleased with him) at the time of Fajr prayer: "Bilal! Tell me about the most hopeful righteous deed you have done, and which, in your view, was for the benefit of Islam, for I heard the sound of your footsteps ahead of me in Paradise last night." He replied: "I do not consider any deed more hopeful to me than this: whenever I have performed ablution at any time of the night or day, I have offered as much voluntary prayer as Allah has destined for me."
Hadith Referenceمسند اسحاق بن راهويه / كتاب التهجد و التطوع / 234
Hadith Takhrij«مسلم ، كتاب فضائل الصحابة رضي الله عنهم ، باب من فضائل بلال رضى الله عنه ، رقم : 2458 . مسند احمد : 439/2 . ابن خزيمه ، رقم : 1208 . مسند ابي يعلي ، رقم : 6104»