在于 (zài yú)

to rest with; to lie in; to be due to (a certain attribute); (of a mat

Character breakdown

在 (zài) · 6 strokes

Composed of 才 and 士, which both represent the sound.
于 (Yú) · 3 strokes

Pictograph of a musical wind instrument with a reed, which is now written as 竽. The current meaning is a phonetic loan.