Я же тебе говорил(а)! (Ya zhe tebye govoril(a)!)
* Я же (Ya zhe) - I, as you know
* тебе (tebye) - to you
* говорил(а) (govoril(a)) - (past tense) said, told
This phrase is used when you are reminding someone of something you previously mentioned, and usually implies that they should have listened to you.
Other ways to say "I told you" in Russian:
* Я предупреждал(а)! (Ya preduprezhdal(a)!) - I warned you!
* Я говорил(а) же! (Ya govoril(a) zhe!) - I told you! (More forceful)
* Как я и говорил(а) ... (Kak ya i govoril(a)...) - As I said...
* Ты сам(а) знаешь! (Ty sam(a) znaesh!) - You know it yourself!
The best way to say it will depend on the specific context and your desired tone.