Map to localization is injective on spectra

This article gives the statement, and possibly proof, of a fact about how a property of a homomorphism of commutative unital rings, forces a property for the induced map on spectra
View other facts about induced maps on spectra

Statement

Suppose $R$ is a commutative unital ring, $U$ is a multiplicatively closed subset of $R$ and $S = U^{- 1} R$ is the localization of $R$ at the multiplicatively closed subset $U$ . Then the induced map on spectra:

$S p e c (S) \to S p e c (R)$

is injective. In fact:

The image of this map is those primes $P$ that are disjoint from $U$
The inverse image of a prime ideal $P$ is precisely the prime ideal $U^{- 1} P$ i.e. the extension of $P$ to $S$