It's not written anywhere. It's from the Hard Rock Treasures DVD. It's his interview near the end with Don Bernstein. He still has black hair.It's around 2005. He actually says that phrase, April. Oh -- he's happy he's kept everything.
http://www.amazon.com/Hard-Rock-Treasures-Ian-Paice/dp/B000FC2GJG/ref=sr_1_1?s=movies-tv&ie=UTF8&qid=1368573419&sr=1-1&keywords=Hard+Rock+Treasures