قُلْ يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَىٰ كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ أَلَّا نَعْبُدَ إِلَّا اللَّهَ وَلَا نُشْرِكَ بِهِ شَيْئًا وَلَا يَتَّخِذَ بَعْضُنَا بَعْضًا أَرْبَابًا مِّن دُونِ اللَّهِ ۚ فَإِن تَوَلَّوْا فَقُولُوا اشْهَدُوا بِأَنَّا مُسْلِمُونَ
“Say, [O Prophet,] 'O People of the Book! Let us come to common terms: that we will worship none but God, associate none with Him, nor take one another as lords instead of God.' But if they turn away, then say, 'Bear witness that we have submitted [to God alone].'”
Per Sahih al-Bukhari, the Prophet's ﷺ letter to Heraclius closes by quoting this ayah, inviting him to "a word common between us and you."
Grouped with the Prophet's ﷺ other letters inviting rulers to Islam (see the Heraclius letter), which classical sources record as closing with this "common word" ayah.