A passage from Epictetus
Epictetus, Enchiridion 18
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When a raven happens to croak unluckily, don't allow the appearance hurry you away with it, but immediately make the distinction to yourself, and say, 'None of these things are portended to me... to me nothing can be portended, either dangerous or unfortunate, since whatever befalls me is up to me.'
Related passages
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Men seek retreats for themselves, houses in the country, sea-shores, and mountains... But this is altogether a mark of the most common sort of men, for it is in thy power whenever thou shalt choose to retire into thyself.
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Take away thy opinion, and then there is taken away the complaint, 'I have been harmed.' Take away the complaint, and the harm is taken away.
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If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment.