It is narrated from Sayyiduna Ali (may Allah be pleased with him) that Witr is not as obligatory as the obligatory prayers (there is no evidence for it from the Qur'an), however, since the Prophet (peace and blessings of Allah be upon him) offered Witr, therefore, O people of the Qur'an! You should also offer Witr, because Allah is Witr (One) and He loves odd numbers.