Quote Quoting cbg
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Dear (Manager):

This letter constitutes my resignation from ABC Company effective April 8, 2019.

Signed,

(Name)

That's all you need.

If, in conversation, you are unable to politely avoid giving a reason, you say, "I'm pursuing another opportunity". Whether or not to add "with DEF Company" is your choice.
Or, in lieu of "I'm pursuing another opportunity", you could just say "family reasons".