It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) in a marfu' form (and we know it only as marfu') that the Prophet (peace and blessings be upon him) said: "Islam is built upon five: bearing witness that there is no true god except Allah, the prayer, and fasting in Ramadan. Whoever abandons any one of them becomes a disbeliever and his blood becomes lawful."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12745