Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "When there is a verse of prostration at the end of a surah, then if you wish, bow (ruku') or prostrate, for in that case the prostration is counted along with the bowing." The narrator said: I asked: O Abu Ishaq, who told you this? He said: Our companions: Alqamah, al-Aswad and al-Rabi' bin Khuthaym.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8680