Abu Sa'id Al-Khudri (May Allah be pleased with him) said

The Prophet (ﷺ) said, "There is a tree in Jannah that is so huge that if a rider of a swift horse has to cover its distance from one end to another in one hundred years, he will not be able to do so."

Another narration from Abu Hurairah (May Allah be pleased with him) is: The Messenger of Allah (ﷺ) said, "A rider will not be able to cross its (the tree's) shade even after travelling for one hundred years."

[Al-Bukhari and Muslim]