Clarification on mass-energy tensor and 4-velocity.

I have next question, we have in the next printcreen the equation:
$$\frac{dU^b}{d\tau}=U^a \nabla_a U^b$$
why is this right?
(the printscreen is from the book of Woodhouse in GR, page 34)

Thanks.
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 Ok, I think I can see why this is right, we pick a frame where $$U^a=(1,0,0,0)$$. Clumsy me. :-)

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