Rukoo' refers to the bowing position which follows the recitation in the prayer.
A person moves to this position by saying takbeer ('Allahu Akbar'), and raising the hands, in the same way that is done at the beginning of the prayer.
The Prophet (may the peace and blessings of Allah be upon him) used to say takbeer when initiating the rukoo'. [al-Bukhārī: 789]
After or during the pronunciation of the takbeer, and before going into the bowing position, the Prophet (may the peace and blessings of Allah be upon him) used to raise his hands, as was narrated by Nāfi' (may Allah have mercy on him):
"Whenever Ibn 'Umar started the prayer, he would say: Allahu Akbar, and he would to raise his hands; and whenever he bowed, he used to raise his hands and also used to raise his hands upon saying, sami' Allahu li-man ḥamidah, and he used to do the same on rising from the second rak'ah. Ibn 'Umar said: 'The Prophet (may the peace and blessings of Allah be upon him) used to do the same.'" [al-Bukhārī: 739]
It was narrated that Mālik ibn al-Ḥuwayrith said:
"I saw the Messenger of Allah (may the peace and blessings of Allah be upon him) raise his hands until they reached the highest part of his ears, when he said Allahu Akbar [to begin the prayer], when he bowed, and when he raised his head from bowing." [an-Nasā'ī: 1024]