See Columbia Accident Investigation Board report, page 127. Hoot Gibson's comments are on p. 31 of the second PDF: http://www.nasa.gov/columbia/caib/PDFS/VOL1/PART02.PDF

Most of the testimony given to the CAIB were sealed (very controversial move), so that's the only quote we have from him. However, he later gave an interview: http://www.spaceflightnow.com/shuttle/sts119/090327sts27/

Another account can be found in Mike Mullane's memoirs, Riding Rockets. Mullane was a mission specialist on STS-27, and operated the Canadarm during the tile inspection. He says that he kept thinking about the tiles, and had trouble sleeping. Hoot Gibson advised him: "No reason to die all tensed up."

Remember -- STS-27 was the second mission after the Challenger explosion. Not enough time yet for complacency to have set in. In addition, Mike Mullane had an it-was-absolutely-not-an-affair-we-were-just-friends relationship with Judy Resnick. So he knew very personally just how dangerous the Shuttle could be.