After rising from bowing, and before going into the position of sujood (prostration), you should say the takbeer, which is to say: "Allaahu akbar", this time without raising the hands. This is an obligatory part of the prayer.

Abu Hurayrah (may Allah be pleased with him) narrated that when the Prophet (may the peace and blessings of Allah be upon him) prayed:

"...he  used to bend for prostration (sujood), after saying Allaahu Akbar...such was the prayer of the Messenger of Allah (may the peace and blessings of Allah be upon him) until he departed from this life."

[al-Bukhārī 803]