In H C Verma,in Newton's Laws chapter, it was given.
The earth in not strictly an inertial frame. However we can say that the earth in an inertial frame of reference to a good approximation.
Thus for routine affairs, acceleration (a) = 0 if and only if external force (F) = 0 is true in the earth frame of reference.
This fact was identified and formualated by Newton as first law(Newton's first law).
Is Newton's first law a law?
If we restrict the statement to measurements made from earth frame, this becomes a law.
If we try to universalize the statement to different frames, it becomes a definition of inertial frame.
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