Alqamah bin Qays and al-Aswad bin Yazid report that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) used to say five takbirs in the first rak'ah, counting the opening takbir and the takbir of bowing, and four takbirs in the second rak'ah, counting the takbir of bowing.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9486