They became close when they both casted in the same musical "Temptation of Wolves"

[TRANS by @mook_yeye]
ZE:A Hyungsik's message to Ryeowook -

@ ZEA_9: Ryeowook hyung!!I'm looking forward to see hyung's comback on the early of next week^_^!! Really want to see you on the Music broadcast program, I guess I will feel completely strangeㅋㅋI will always support you, so grateful and admire you! Super Junior sunbaenim's comback!!Completely look forward~~Hwaiting!^_^

@ ZEA_9: 려욱형!!난 담주 형의 컴백만을 기다리고있다구요^_^!! 너무 보고싶어요 음악방송에서 만나면 완전 기분 이상할것같아요ㅋㅋ형 항상 응원해주셔서 너무 고맙고 존경해요! 슈퍼주니어선배님들의 컴백!!완전 기대하고있습니다~~화이팅!^_^

Source: ZEA_9 twitter