The Prophet ﷺ said, "The truest line of poetry ever spoken by a poet is, 'Indeed, everything besides Allah is false.' And Ibn Ubayy as-Salt was on the verge of becoming Muslim."
حدثنا سفيان عن زائدة عن عبد الملك بن عمير عن أبي سلمة
عن النبي ﷺ أصدق بيت قاله الشاعر ألا كل شيء ما خلا الله باطل وكاد ابن أبي الصلت يسلم
Its isnad is sahih. Zaida is Ibn Qudama Thaqafi, whose reliability was already established at 1067, and we add here that Bukhari mentioned him in Al-Kabir 1/395/2, and Ibn Abu Hatim in 1/2/613. The hadith was narrated by Muslim 2:198, from Ibn Umar, and by Ibn Majah 3757, from Muhammad bin Sabbah, both of them from Sufyan bin Uyayna, with the same wording. Bukhari also narrated it at 115-116, and 10:448, and 11:275, and Muslim also at 2:198-199, with similar wording, both at length and in abridged form, through other routes. See also what came earlier in the Musnad of Ibn Abbas, 2314.