I wanted to ask is the following oft-cited narrative, ascribed to the Prophet (pbuh) is authentic? "Seek knowledge even (if you have to go to) China." Some scholars maintain that it is not a Hadith. Does any reliable book of hadith quote it?
This narrative does not appear in any reliable Hadith collection and finds mention only in some of the less authoritative works. Different scholars have recorded it from a very weak and unreliable chain of narrators. The Sanad (chain of narrators) and Matn (text) of the Ḥadīth, as quoted in Allamah Ibn Abdul Barr's Jami' Bayaan al 'ilm, follows:
اخبرنا احمد حدثنا مسلمة حدثنا يعقوب بن اسحاق بن ابراهيم العسقلاني حدثنا يوسف بن محمد الفريابي حدثنا سفيان بن عيينة عن الزهري عن انس بن مالك قال: قال رسول الله صلى الله عليه و سلم:اطلبوا العلم و لو بالصين فان طلب العلم فريضة على كل مسلم
Ahmed narrates from Muslamah who narrates from Ya'qub b. Ibrahim al Asqalani who narrates form Yusaf b. Muhammad al Faryaabi who narrates from Sufyaan b. 'Uyayna who narrates form Al-Zuhri who narrates from Anas b. Malik that the Prophet (pbuh) said: Seek knowledge even [you have to go to] China, for seeking knowledge is a duty binding on every Muslim.
According to the authorities, the narrative is a fabrication. Ibn al Jawzi in his work al-Mawzu'at, 'Allamah Dhabi, in his work Tarteeb-al-Mawzu'at and Allamah Nasir Uddin Albaani in his Silsilah Ahadith Zaheefah commented that the narrative is a fabrication and has no basis in the sources.
 al-Mazoo'aat vol. 1, page 215, Tarteeb-al-Mawzoo'aat page 111 and Silsilah Ahadith Mawzoo'ah narrative no: 906.