Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated from Sayyiduna Abu Hurairah and Sayyiduna Zayd bin Khalid, may Allah be pleased with them) that the Messenger of Allah, peace and blessings be upon him, was asked about a slave-girl who commits adultery and she is not muhsanah (not married), so the Prophet, peace and blessings be upon him, said: "When she commits adultery, flog her; then if she commits adultery, flog her; then if she commits adultery, flog her; then sell her even if (her price) is «ضيفر» (a rope)." Zuhri said: I do not know whether the Prophet said this the third time or the fourth time? And «ضيفر» means a rope.