My iPhone heats up without any reason

If you have considered all the information in the link above your post then the next step would be to contact Apple directly.

https://support.apple.com/

sarath ~ What model of iPhone is it?...

Settings > General > About > Model Name

And what version of iOS is it running?...

Settings > General > About > iOS Version

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I’ve seen this suggestion in the forums. Try restarting your iPhone. Please let us know if that helps.

Restart your iPhone - Apple Support