PHB, page 55.
Skills: The character retains and can access skills from all his or her classes. For purposes of calculating maximum ranks, a skill is a class skill if at least one of the character’s classes has it as a class skill. The maximum rank for a class skill is 3 + the character level. For a 7th-level rogue/4th-level wizard (an 11th-level character), a rogue skill or wizard skill has a maximum rank of 14. For a cross-class skill, maximum rank is half the maximum for a class skill.
If a skill is unavailable to a class (that is, if it’s an exclusive skill that a multiclass character’s other class doesn’t have access to), then levels in that class don’t increase the multiclass character’s maximum ranks. For instance, the 7th-level rogue/4th-level wizard would have a maximum rank of 10 in Use Magic Device, an exclusive class skill for rogues. The extra four levels in the wizard class do not increase the character’s maximum rank with Use Magic Device. Likewise, the same character could have a maximum of 7 ranks in Scry, an exclusive skill for wizards.