Source, in full
As quoted in the epigraph in Fahrenheit 451 (1953) by Ray Bradbury; Susie Salmon also uses this quote in The Lovely Bones, and Daniel Quinn published a book in 2007 with the title If They Give You Lined Paper, Write Sideways (2007)
Notes from Wikiquote
Spanish: "Si os dan papel pautado, escribid por el otro lado" (If they give you lined paper, write on the other side) | Note: "If they give you ruled paper, write the other way" is often attributed to William Carlos Williams who was contemporary with JRJ.