You need to get a reference to the body element of the TinyMCE iframe content area. After that you can implement a custom text source similarly as it is done for RadEditor in the following forum thread on the subject: Problem with spellcheck
Using the RadEditor's get_contentAreaElement() you can get a reference to RadEditor's iframe content area element using this code to get a reference to the body tag of the iframe:
var editorSource = $find('RadEditor1').get_contentAreaElement().contentWindow.document.body;
You should see in the TinyMCE documentation or forums how to get a reference to the body element and update the above line in the provided in the forum article text source code.
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