“then if they turn away, say: ‘Bear witness that we are Muslims.’” (3:64)

This is the very verse the Prophet—may Allah bless him and grant him peace—quoted in the letter he sent to Caesar, the ruler of the Byzantines, when he invited him to Islam. In that letter he wrote:

“In the name of Allah, the All-Merciful, the Ever-Merciful.
From Muhammad, the Messenger of Allah, to Heraclius, the great leader of the Byzantines.
Peace be upon whoever follows guidance.
To proceed:
I invite you with the call of Islam. Accept Islam and you will be safe; accept Islam and Allah will grant you your reward twice over. But if you turn away, the sin of the arīsiyyūn (your common subjects) will rest upon you. And [Allah says]:
‘People of the Scripture, come to a word that is just between us and you: that we worship none but Allah, associate nothing with Him, and that none of us takes others as lords besides Allah. If they turn away, then say: Bear witness that we are Muslims.’ (3:64)”