According to White Hinterland, an American singer, the duo used a vocal sample from her tack 'Ring The Bell'. She recently filed a lawsuit sighting copyright infringement and took to Facebook to outline the situation. Apparently she reached out to Bieber's team in the past, but was ignored.
"As many of you that follow my career and work have already recognized, Justin Bieber’s song 'Sorry' copies the vocal riff prominently featured in my song 'Ring the Bell.' The writers, producers, and performers of 'Sorry' did not obtain a license for this exploitation of my work, nor did they obtain or seek my permission.
"Like most artists that sample music, Bieber could have licensed my song for use in 'Sorry.' But he chose not to contact me. After the release of 'Sorry,' my lawyers sent Bieber a letter regarding the infringement, but Bieber’s team again chose to ignore me. I offered Bieber’s team an opportunity to have a private dialogue about the infringement, but they refused to even acknowledge my claim, despite the obviousness of the sample."
The subtle vocal on 'Sorry', which was produced by Skrillex and released on Bieber's album Purpose, is somewhat recognizable as being influenced by White Hinterland's track 'Ring The Bell'. Listen to both tracks below.