From Sahih Bukhari (Hadith 6613):
Narrated Ibn `Abbas:(regarding the Verse) "And We granted the vision (Ascension to the heavens "Miraj") which We showed you (O Muhammad as an actual eye witness) but as a trial for mankind.' (17.60): Allah's Apostle actually saw with his own eyes the vision (all the things which were shown to him) on the night of his Night Journey to Jerusalem (and then to the heavens). The cursed tree which is mentioned in the Qur'an is the tree of Az-Zaqqum
Reference: Bukhari 6613