Juz RafaYadain is written by imam bukhari where he collect more than 100 hadees on rafayadan

Little about Iman Bukhari
Muhammad ibn Ismail al-Bukhari al-Jufi was born after the Jumu'ah prayer on Friday, 19 July 810 (13 Shawwal 194 AH) in the city of Bukhara in Transoxiana (in present-day Uzbekistan)

His father, Ismail ibn Ibrahim, a scholar of hadith, was a student and associate of Malik ibn Anas. Some Iraqi scholars related hadith narrations from him.
Juz rafa ul yadain is written by Imam bukhari when he realizes that people become lazy on doing rafayadain during prayer and some people says its not necessary.
He collect more that 100 sahih ahadees on rafa ul yadain to prove that it is Part of Prayer and still Necessary.