Working with Barbra Streisand on her duets album, 'Partners,' left Blake Shelton speechless ... something he doesn't experience often. To thank him, she sent him a gift -- and it's one that he says is the best he's ever received.

Shelton couldn't believe it when Streisand called him up and asked him to join her on 'I'd Want It to Be You.' He tells People his only words were "Who? What? How?" He isn't often left speechless -- or shocked, for that matter -- but that phone call left him feeling both.

"It's still shocking to me. I don't know how it came down," he says. "I don't know how she knows about me, but I'm just glad. I'm just going with it!"

He's probably not the only one "glad" about how things turned out. 'Partners' debuted at No. 1, which makes Streisand the only singer to achieve such a feat in each decade for the last six decades. Shelton wasn't entirely happy about the recording, however, as he didn't actually get to cut his lines with the legendary songstress in person.

"Ironically enough, she had already done her vocal part and I was really disappointed because when I went to the studio, she wasn't there," the country star explains. "I was like, 'Oh, man, we ... that's ... I still get to be on her record with her and that's bada--.' "

Shelton says the moment was definitely "nerve-wracking," but said Streisand was "humble" and "appreciative" of what he did. She showed her appreciation with a pretty fancy gift, one Shelton says is the best thing on his mantle.

"She sent me a piece of crystal that was like, 'Thank you, Love Barbra.' I pushed all my awards away and put that in the middle," he insists. "It was so cool, like, 'See, she knows me. I knew it.'"

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