Remark 2
Tag 0ANY, permanent. Cite it as [Bourbaki, Tag 0ANY].
Lie Groups and Lie Algebras, chapter IV, § 2. Tits Systems
Suppose that Z \cap U = \{1\}. Since Z and U are normal in B, it follows that every element of Z commutes with every element of U, and hence with every element of G_1. In view of the preceding corollary, it follows that G_1 \cap Z is the centre of G_1.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.
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