كتاب الإيمان
1  
The Book of Faith
(10)
Chapter: The evidence that one who dies believing in tawhid will definitely enter paradise
(10)
باب مَنْ لَقِيَ اللَّهَ بِالإِيمَانِ وَهُوَ غَيْرُ شَاكٍّ فِيهِ دَخَلَ الْجَنَّةَ وَحَرُمَ عَلَى النَّارِ
Sahih Muslim 28 a

It is narrated on the authority of Ubadah b. Samit that the messenger of Allah (ﷺ) observed:

He who said:" There is no god but Allah, He is One and there is no associate with Him, that Muhammad is his servant and His messenger, that Christ is servant and the son of His slave-girl and he (Christ) His word which He communicated to Mary and is His Spirit, that Paradise is a fact and Hell is a fact," Allah would make him (he who affirms these truths enter Paradise through any one of its eight doors which he would like.
حَدَّثَنَا دَاوُدُ بْنُ رُشَيْدٍ، حَدَّثَنَا الْوَلِيدُ، - يَعْنِي ابْنَ مُسْلِمٍ - عَنِ ابْنِ جَابِرٍ، قَالَ حَدَّثَنِي عُمَيْرُ بْنُ هَانِئٍ، قَالَ حَدَّثَنِي جُنَادَةُ بْنُ أَبِي أُمَيَّةَ، حَدَّثَنَا عُبَادَةُ بْنُ الصَّامِتِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مَنْ قَالَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ وَأَنَّ عِيسَى عَبْدُ اللَّهِ وَابْنُ أَمَتِهِ وَكَلِمَتُهُ أَلْقَاهَا إِلَى مَرْيَمَ وَرُوحٌ مِنْهُ وَأَنَّ الْجَنَّةَ حَقٌّ وَأَنَّ النَّارَ حَقٌّ أَدْخَلَهُ اللَّهُ مِنْ أَىِّ أَبْوَابِ الْجَنَّةِ الثَّمَانِيَةِ شَاءَ ‏"‏ ‏.‏
Reference : Sahih Muslim 28 a
In-book reference : Book 1, Hadith 47
USC-MSA web (English) reference : Book 1, Hadith 43
  (deprecated numbering scheme)