Zirr bin Hubaysh (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the obligatory prostrations are four: in Surah al-Sajdah, Surah Ha-Mim al-Sajdah, Surah al-Alaq and Surah al-Najm.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3715