Grading: (1) The marfu' part at its end is "hasan li-ghayrihi" (sound due to supporting reports). This chain has some softness (lin) on account of Shahr bin Hawshab.
Technical note / Defect ('illah): But Abd al-Hamid bin Bahram narrated this story from him, with Ahmad [3/ (21724)], and his narration from Shahr is strong according to some scholars. Perhaps that is why al-Dhahabi judged its chain good (jayyid) in his "Talkhis".

Reference / Source: In another narration, through the route of Qatadah from Shahr bin Hawshab in mursal form, about the story of Abu Dharr, in Ahmad's "al-Zuhd" (798), there is something that explains the statement here about Abu Dharr being sent to al-Rabadhah: that it was by Abu Dharr's own choice and request, after Uthman bin 'Affan (may Allah be pleased with him) had offered him to stay in Madinah. Uthman said to him: "O Abu Dharr, stay with us; milch camels will come to you morning and evening." He said: "I have no need of them", and said: "Al-Rabadhah was my dwelling place, so permit me to go to it." So he permitted him.

Corroborating chains and supporting reports: This is supported by the hadith of Zayd bin Wahb with al-Bukhari (1406), who said: I passed by al-Rabadhah and there was Abu Dharr. I said to him: What brought you to this place of yours? He said: I was in Syria (and he mentioned his story with Mu'awiyah about their disagreement over hoarding gold and silver, and that Mu'awiyah complained about him to Uthman, so Uthman summoned him to Madinah). Abu Dharr said: Then the people crowded around me as though they had never seen me before. I mentioned that to Uthman, and he said to me: "If you wish, you may withdraw and yet be near." That is what brought me to this place.

Corroborating chains and supporting reports: And the hadith of Abd Allah bin al-Samit with Ibn Sa'd (4/ 218), Ibn Shabbah in "Tarikh al-Madinah" (3/ 1035), Abu 'Awanah in his "Sahih" (11467 - Islamic University edition) and Ibn Hibban (5964), about the story of Abu Dharr coming to Uthman when he arrived from Syria. In it: he asked his permission to go to al-Rabadhah and he permitted him; indeed Uthman said to him: "We permit you, and we order for you some of the camels of the sadaqah, so that you may have their milk (rislaha)."
Note / Clarification: "Al-risl": milk. The mursal narration of Muhammad bin Sirin [Ibn Sa'd 4/ 212, and Ibn Abi 'Umar al-'Adani in his "Musnad" as in al-Busiri's "Ithaf al-Khiyarah" (4178)] is also like the narration of Qatadah from Shahr, and its narrators are thiqah (trustworthy). It has an addition similar to the hadith of Abu Dharr that comes after this.

Note / Clarification: Its marfu' part has already passed through another route from Abu al-Darda' under number (5552).