Doesn't push build.prop?!

HI again. I have ADB pulled build.prop file to change something. Now, I am finished. Last step is to push the file to the Android device that I have. OK. I write

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adb push build.prop /system/build.prop

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then it says

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failed to copy "build.prop" to "/system/build.prop" : Permission denied

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What's the problem? Thanks