Majma al-Zawaid 3712 — A third chapter on the same subject

It is also reported from him that he said: When the prostration is at the end of the surah, then if you wish, go into bowing, or else go into prostration; the prostration is together with the bowing.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are thiqat (trustworthy).