You are correct, Knight of Gondor. Saruman had Radagast deliver the message to Gandalf that Gandalf should "seek [Saruman's] aid at once." That is the simple explanation for why Gandalf went.
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until Gandalf came to Isengard, and told Saruman that the Ring was in the hands of a Hobbit.(Finwe)
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In the books, Gandalf never tells Saruman that the Ring is in the hands of a hobbit.