It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Witr is a truth (established duty)", then the narrator mentioned the hadith with a meaning similar to the previous one. Imam al-Hakim says: I have no doubt that the two Shaykhs left this hadith only because some of the students of Imam al-Zuhri narrated it as mawquf (stopping at the Companion), whereas this is not something that creates a scholarly hidden defect ('illah) or weakness in a hadith like this. And Allah knows best.