Ibn Mas'ud told that when Ibn an-Nawwaha and Ibn Uthal, Musailima’s messengers, came to the Prophet (ﷺ) he asked them whether they testified that he was God’s Messenger. On their replying, "We testify that Musailima is God’s messenger,” he said, "I believe in God and His Messenger. If it was my custom to kill a messenger I would kill you.” ‘Abdallah (i.e. Ibn Mas'ud) said that the sunna that a messenger should not be killed then came into force. Ahmad transmitted it.
Source: sunnah.com
Other translations
Arabic
عَنِ ابْنِ مَسْعُودٍ قَالَ: جَاءَ ابْنُ النَّوَّاحَةِ وَابْنُ أُثَالٍ رَسُولَا مُسَيْلِمَةَ إِلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ لَهُمَا: «أَتَشْهَدَانِ أَنِّي رَسُولُ اللَّهِ؟» فَقَالَا: نَشْهَدُ أَنَّ مُسَيْلِمَةَ رَسُولَ اللَّهِ فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «آمَنْتُ بِاللَّهِ وَرَسُولِهِ وَلَوْ كُنْتُ قَاتِلًا رَسُولًا لَقَتَلْتُكُمَا» . قَالَ عَبْدُ اللَّهِ: فَمَضَتِ السُّنَّةُ أَنَّ الرَّسول لَا يُقتَلُ. رَوَاهُ أَحْمد
sunnah.com