B1A4's Sandeul shared the story behind MAMAOO Whee In's featuring in the song "Hey!' from his solo album 'Stay As You Are'.

On the October 4th airing of 'Kim Shin Young's Hope Song at Noon', Sandeul introduced his album, saying, "There's a song featuring MAMAMOO's Whee In. It's a song about young love." He then complimented Whee In by adding, "I was so surprised because she can sing so well. I hope many people realize her singing talent." 

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Kim Shin Young then asked Sandeul how Whee In came to work with him, and he answered, "MAMAMOO's Moon Byul is a friend in the '92 club.' I listen to MAMAMOO's songs often because I'm close to her, and out of all, Whee In's voice stood out. I came to think that it'll be nice to have her feature in my song. So I told Moon Byul, and she [Whee In] willingly agreed. I'm thinking to pay back to Moon Byul with delicious food soon."