"Witness by Katy Perry on Apple Music"(Credits can be obtained from the songs once added to the iTunes library). iTunes Store. 16 Haziran 2017 tarihinde kaynağından arşivlendi. Erişim tarihi: 3 Haziran 2017.
"Czech Albums – Top 100". ČNS IFPI. Note: On the chart page, select {{{date}}} on the field besides the word "Zobrazit", and then click over the word to retrieve the correct chart data. Erişim tarihi: 20 Haziran 2017.
"Witness by Katy Perry on Apple Music"(Credits can be obtained from the songs once added to the iTunes library). iTunes Store. 16 Haziran 2017 tarihinde kaynağından arşivlendi. Erişim tarihi: 3 Haziran 2017.