He traced it back to the Prophet ﷺ, who said, "In Paradise there is a tree such that a rider could travel in its shade for a hundred years and still not cross it. Recite if you wish, 'And extended shade' [Waqiah 56:30]."
يبلغ به النبي ﷺ قال: إن في الجنة شجرة يسير الراكب في ظلها مائة عام، لا يقطعها، واقرؤوا إن شئتم: {وظل ممدود} [سورة الواقعة: ٣٠].
Bukhari narrated it in the tafsir (4881), and Muslim in the chapter on Paradise (7:2826), both through the route of Abu Zinad, from Araj, from Abu Hurairah, as he mentioned it. The wording given is that of Bukhari, for Muslim did not state its wording with this chain, but rather referred back to the hadith preceding it.