Proposition 10
Tag 08SS, permanent. Cite it as [Bourbaki, Tag 08SS].
Integration, chapter IV, § 4. Integrable functions and sets
— For an open (resp. closed) set in to be integrable, it is necessary and sufficient that .
Since is then lower (resp. upper) semi-continuous, the proposition follows from Prop. 5 of No. 4 and its Corollary 1.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.