Sahih Bukhari Volume 6, Book 65, Hadith 4718

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرٍو، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنه ـ ‏{‏وَمَا جَعَلْنَا الرُّؤْيَا الَّتِي أَرَيْنَاكَ إِلاَّ فِتْنَةً لِلنَّاسِ‏}‏ قَالَ هِيَ رُؤْيَا عَيْنٍ أُرِيَهَا رَسُولُ اللَّهِ صلى الله عليه وسلم لَيْلَةَ أُسْرِيَ بِهِ ‏{‏وَالشَّجَرَةَ الْمَلْعُونَةَ‏}‏ شَجَرَةُ الزَّقُّومِ‏.‏

Narrated By Ibn Abbas : Regarding: 'And We granted the vision (Ascension to the Heaven "Miraj") which We showed you (O Muhammad as an actual eye witness) but as a trial for mankind.' (17.60) It was an actual eye-witness which was shown to Allah's Apostle during the night he was taken on a journey (through the heavens). And the cursed tree is the tree of Az-Zaqqum (a bitter pungent tree which grows at the bottom of Hell).