Sayyiduna Ali (may Allah be pleased with him) said that Witr does not have definitive proof from the Noble Qur’an like the obligatory prayers, but its obligation is established from the Sunnah of the Prophet (peace and blessings be upon him), and Allah Almighty is Odd and loves the odd number.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 786
Hadith Gradingحکم دارالسلام:صحيح، أبو خيثمة - وإن كان سماعه من أبى إسحاق بعد الاختلاط - قد توبع