ENOfficial
It was narrated that Jarir said: "Any slave who runs away from his masters and joins the enemy, he has made it permissible to shed his blood."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ حَدَّثَنَا شَرِيكٌ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَامِرٍ، عَنْ جَرِيرٍ، قَالَ أَيُّمَا عَبْدٍ أَبَقَ مِنْ مَوَالِيهِ وَلَحِقَ بِالْعَدُوِّ فَقَدْ أَحَلَّ بِنَفْسِهِ .
sunnah.com