Grading: Its chain is "da'if" (weak).
Technical note / Defect ('illah): Because al-Hasan (al-Basri) did not hear this hadith from Samurah, as is explicitly stated in the narration of Shu'bah in Musnad Ahmad (20104) (from Qatadah, from al-Hasan, from Samurah), (that there was no hearing): he did not hear it from him, that "the Messenger of Allah (peace and blessings be upon him) said" (and then he mentioned the hadith).

Reference / Source: Ahmad (33/20197) narrated it from Yazid bin Harun, with this same chain.

Reference / Source: Ahmad (20104, 20125, 20132, 20137, 20214), Abu Dawud (4515, 4516, 4517), Ibn Majah (2663), al-Tirmidhi (1414) and al-Nasa'i (6912, 6913, 6914, 6929, 6930) narrated it through various routes from Qatadah, from al-Hasan.
Details of the narration: Abu Dawud in narration (4516) and al-Nasa'i in narrations (6912, 6930) added: "And whoever castrates his slave, we will castrate him." (The author, al-Hakim, will bring these words below in narration no. 8299.)
Important point: In Ahmad's narration (20214) and Abu Dawud's narration (4517) there is also this addition: "Then al-Hasan (al-Basri) forgot this hadith, and he used to say: A free man is not killed for a slave." Imam al-Tirmidhi said: This hadith is "hasan gharib".

General principle: Imam al-Tirmidhi says: Some of the people of knowledge among the Successors, including Ibrahim al-Nakha'i, held this view (that a free man is killed for a slave). Some of the people of knowledge, including al-Hasan al-Basri and Ata' bin Abi Rabah, say: "Between a free man and a slave there is no retaliation (qisas), neither for a life nor for what is less than a life (injuries to limbs)." This is the view of Ahmad and Ishaq. Some said: "If he kills his own slave he is not killed (in retaliation) for him, but if he kills someone else's slave he is killed for him." This is the view of Sufyan al-Thawri and the people of Kufah.

General principle: Al-Baghawi says in "Sharh al-Sunnah" (10/178): "The general body of the people of knowledge hold that a limb of a free man is not cut off in retaliation for the limb of a slave. This agreement establishes that the hadith is to be understood as a deterrent and restraint, or else it is abrogated."

Reference / Source: For the statements of the people of knowledge on this issue see: al-Baghawi's "Sharh al-Sunnah" (10/177-178), Ibn Qudamah's "al-Mughni" (11/474-475) and Abu Ubayd's "al-Nasikh wa'l-Mansukh" (p. 139).