Corollary 3
Tag 096D, permanent. Cite it as [Bourbaki, Tag 096D].
Integration, V, p. 76, § 7. Integration with respect to an induced measure
Let be a complex measure on T; then
Set and apply Cor. 2 on taking to be a complex function of absolute value 1 such that (§ 5, No. 5, Cor. 3 of Th. 2); then ; but is a function of absolute value 1, and the formula (6) follows from Prop. 2 of § 5, No. 2.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.