Hadith 1132

'Ali (bin Abu Talib) (May Allah be pleased with him) reported: The Witr prayer is not obligatory as the prescribed Salat (prayers), but the Messenger of Allah (PBUH) observed it as his regular practice (Sunnah). He (PBUH) said, "Allah is Witr (single, odd) and loves what is Witr. So perform Witr prayer. O followers of Qur'an, observe Witr (prayer)."

[At-Tirmidhi and Abu Dawud].