Riyad as-Salihin, Book 1, Hadith 242 (In-Book) ⓘ

Abu Hurairah (May Allah bepleased with him) reported: The Prophet said, "When a slave-girl commits fornication and this fact of fornication has become evident, she must be given the penalty of (fifty) lashes without hurling reproaches at her; if she does it again, she must be given the penalty but she should not be rebuked. If she does it for the third time, he should sell her, even for a rope woven from hair (i.e., something worthless)".[Al-Bukhari and Muslim].

وعنه النبي صلى الله عليه وسلم قال‏:‏ ‏"‏إذا زنت الأمة فتبين زناها فليجلدها الحد، ولا يثرب عليها، ثم إن زنت الثانية فليجلدها الحد ولا يثرب عليها، ثم إن زنت الثالثة فليبعها ولو بحبل من شعر‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏ ‏.‏ (15)