The Prophet ﷺ said: If any slave marries without the permission of his masters, he is a fornicator. (Using translation from Abū Dāʾūd 2078)  

قَالَ رَسُولُ اللَّهِ ﷺ «أَيُّمَا عَبْدٍ تَزَوَّجَ بِغَيْرِ إِذْنِ مَوَالِيهِ أَوْ أَهْلِهِ فَهُوَ عَاهِرٌ»  

Abdullah narrated that the Prophet said: "Whichever slave gets married without the permission of his owner, then he is a fornicator." (Using translation from Tirmidhī 1112)   

عَنِ النَّبِيِّ ﷺ قَالَ «أَيُّمَا عَبْدٍ تَزَوَّجَ بِغَيْرِ إِذْنِ مَوَالِيهِ فَهُوَ زَانٍ»