A man said to Ibn Umar, "Abu Hurayrah, may Allah be pleased with him, claims that the witr prayer is not obligatory."Ibn Umar replied, "A man asked the Messenger of Allah ﷺ about the night prayer, and he said, 'The night prayer is offered in pairs. When you fear the approach of dawn, pray a single rakah as witr.'"
حدثنا جرير عن منصور عن حبيب
قال رجل لابن عمر إن أبا هريرة رضي الله تعالى عنه يزعم أن الوتر ليس بحتم قال سأل رجل رسول الله ﷺ عن صلاة الليل فقال صلاة الليل مثنى مثنى فإذا خفت الصبح فأوتر بواحدة
Its isnad is sahih. Mansur is Ibn Muatamir. Habib is Ibn Abu Thabit, and he did hear from Ibn Umar, but he did not hear this particular hadith from him, so he related it from him through Tawus.This hadith has already appeared many times with the same meaning, namely that night prayer is performed two rakats at a time, and that Witr is one rakat before Fajr, including at 6176. There also already appeared a man's question to Ibn Umar about Witr, asking whether it was a sunnah, at 4834, and his question about it, asking whether it was obligatory, at 5216.Muslim related, 1:208, the hadith "night prayer is performed two rakats at a time" through the narration of Amr bin Dinar from Tawus from Ibn Umar, and Bayhaqi likewise related it, 3:22, by the route of Amr bin Dinar from Tawus. But I have not found this particular wording that appears here, from the narration of Habib bin Abu Thabit from Tawus, except in this one place. See also 6190.The word "hatm," with a fatha on the ha and a sakin on the ta, means that which is necessary and obligatory, something that must be done.