Rapper Sleepy showed off his playful bond with Baek Ah Yeon on Sunday via Instagram when he called the singer "noona," or older sister.

On October 18th, Sleepy snapped a photo with Baek thanking her for lending her voice for his song. Even though the rapper is nine years older than Baek Ah Yeon, it just shows how comfortable the two got with their recent collaboration.

"Ah Yeon noona, thank you for featuring [on my song]," posted Sleepy on his Instagram on Sunday.

The photo showed Sleepy and Baek posing together in the recording room.

Sleepy will be releasing his first solo single "F/W" on Tuesday.