From Menorat Zahav, Parshat Ha’azinu.
When I call out the LORD’s name, give glory to our God. Deuteronomy 32:3
In the book of Isaiah it is written, “I am first and I am last” (1). Rebbe Tzvi Mendel [Rebbe Zusya’s son] explained that only God can claim to be “I,” and before we say God’s holy name we must awaken ourselves in all our limbs, as God says in praise of Levi, “he stood in awe of My name” (2)… It should be the same after we speak God’s name. Our sages compare speaking God’s name to drinking fine spiced wine, for the flavor remains in your mouth (3). This is a test that we have prayed genuinely. This is the meaning of “I am first and I am last” — we should both experience the awe of God before saying God’s name as well as after.
This is how we should understand this verse [which we say before the afternoon Amidah prayer].
1) Isaiah 44:6 2) Micah 2:5; Levi was the tribe whose members served in the Tabernacle and then the Temple, and so represents proper prayer. 3) Yerushalmi Shekalim 2