ENOfficial
Nu'aim b. Mas'ud reported God's Messenger as saying to two men who came from Musailima, "I swear by God that were it not that messengers are not killed, I would cut off your heads.” Ahmad and Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ نُعَيْمِ بْنِ مَسْعُودٍ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ لِرَجُلَيْنِ جَاءَا مِنْ عِنْدِ مُسَيْلِمَةَ: «أَمَّا وَاللَّهِ لَوْلَا أَنَّ الرُّسُلَ لَا تُقْتَلُ لَضَرَبْتُ أَعْنَاقَكُمَا» . رَوَاهُ أَحْمَدُ وَأَبُو دَاوُد
sunnah.com