Abd al-Rahman bin Abi Amrah al-Bukhari asked Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) about witr, and he said: It is a very good deed; the Prophet (peace and blessings be upon him) acted upon it, and the Muslims after him, but it is not obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4452