The Quran

وَإِذا قيلَ لَهُم آمِنوا بِما أَنزَلَ اللَّهُ قالوا نُؤمِنُ بِما أُنزِلَ عَلَينا وَيَكفُرونَ بِما وَراءَهُ وَهُوَ الحَقُّ مُصَدِّقًا لِما مَعَهُم ۗ قُل فَلِمَ تَقتُلونَ أَنبِياءَ اللَّهِ مِن قَبلُ إِن كُنتُم مُؤمِنينَ

Muhammad Asad

For when they are told, "Believe in what God has bestowed from on high," they reply, "We believe [only] in what has been bestowed on us" - and they deny the truth of everything else, although it be a truth confirming the one already in their possession. Say "Why, then, did you slay God's prophets aforetime, if you were [truly] believers?"¹

A reference to their assertion that they believe in what has been revealed to them - i.e., the Law of Moses, which obviously prohibits the killing not only of prophets but of any innocent human being. See also the concluding sentences of verses 61 and 87, and the corresponding notes.