I say: It appears that al-Haythamī withheld judgment about him and therefore remained silent, even though the scholars differ over his reliability. A narrator of his standing is deemed to produce a good (ḥasan) report so long as he does not contradict stronger authorities—especially when he is not alone in transmitting it—and this ḥadith is of that sort.
Al-Tirmidhī recorded it (2/131) through the route of Khālid ibn Ilyās—also called Ibn Iyās—from Ṣāliḥ ibn Abī Ḥassān, who said: “I heard Saʿīd ibn al-Musayyib say: ‘Allah is Good and loves goodness, Clean and loves cleanliness, Generous and loves generosity, Bountiful and loves bounty; so cleanse—I think he said—your courtyards and do not resemble the Jews.’” Ṣāliḥ continued: “I mentioned that to Muhājir ibn Mismār, and he said: ‘ʿĀmir ibn Saʿd narrated it to me from his father, from the Prophet ﷺ in the same wording, except that he said: Cleanse your courtyards.’”
Al-Tirmidhī commented: “This is a gharīb (singular) ḥadith, and Khālid ibn Ilyās is weak.”
I add: In al-Taqrīb he is described as “matrūk al-ḥadīth (abandoned).”
Ibn al-Qayyim quoted the report in Zad al-Maʿād (3/208): “In the Musnad of al-Bazzār the Prophet ﷺ said: ‘Allah is Good …’—to the end of the ḥadīth—‘so cleanse your yards and open spaces and do not imitate the Jews; they gather heaps of refuse inside their houses.’”
I do not know whether al-Bazzār relates it through this same Khālid or by another route.
I have come across still another chain for it, though it brings little joy. Al-Dūlābī narrated it in al-Kunā (2/16) from Abū al-Ṭayyib Hārūn ibn Muḥammad, who said: “Bukayr ibn Mismār narrated to us from ʿĀmir ibn Saʿd …” and so forth. All the transmitters in this chain are trustworthy—except for Abū al-Ṭayyib, who is anything but good! Ibn Maʿīn said: “He was a liar.”