Sahih Muslim 16 a
It is narrated on the authority of ('Abdullah) son of Umar (may Allah be pleased with them) that the Prophet (may peace of Allah be upon him) said:
(The superstructure of) al-Islam is raised on five (pillars), i. e. the oneness of Allah, the establishment of prayer, payment of Zakat, the, fast of Ramadan, Pilgrimage (to Mecca). 
A person said (to 'Abdullah b. Umar the narrator): Which of the two precedes the other-Pilgrimage or the fasts of Rarnadan? Upon this he (the narrator) replied: No (it is not the Pilgrimage first) but the fasts of Ramadan precede the Pilgrimage.
حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ الْهَمْدَانِيُّ، حَدَّثَنَا أَبُو خَالِدٍ، - يَعْنِي سُلَيْمَانَ بْنَ حَيَّانَ الأَحْمَرَ - عَنْ أَبِي مَالِكٍ الأَشْجَعِيِّ، عَنْ سَعْدِ بْنِ عُبَيْدَةَ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏
"‏ بُنِيَ الإِسْلاَمُ عَلَى خَمْسَةٍ عَلَى أَنْ يُوَحَّدَ اللَّهُ وَإِقَامِ الصَّلاَةِ وَإِيتَاءِ الزَّكَاةِ وَصِيَامِ رَمَضَانَ وَالْحَجِّ ‏"‏ ‏.‏ فَقَالَ رَجُلٌ الْحَجِّ وَصِيَامِ رَمَضَانَ قَالَ لاَ ‏.‏ صِيَامِ رَمَضَانَ وَالْحَجِّ ‏.‏ هَكَذَا سَمِعْتُهُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم ‏.‏