Musannaf Ibn Abi Shayba 23:73

Waki’ narrated to us, from ‘Amr ibn Munabbih, from Siwar ibn Shabib, who said: A man came to Ibn ‘Umar and said: "There are people here who testify against me with disbelief." He said: "Will you not say 'There is no god but Allah,' and thus prove them liars?"