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They are most commonly attributed to Hajjaaj ibn Yusuf, under the reign of Abdul Malik around 65-85 hijri.
The sign of Ruku’ or Section is to identify or determine the average portion of verses which could be recited in one unit of salah (raka'ah), and ruku' could be made in salah after reaching this point. It is placed, as far as possible, at the conclusion of a statement. There are 540 ruku’at in the whole Qur’an. So, by reciting one ruku’ in every raka’ah of tarawih, it is possible to complete the recital of the entire Qur’an on the night of the twenty seventh day.
From:
http://www.as-sidq.org/download/quran.pdf
Mufti Taqi Usmani says:
Inspite of his efforts, this humble writer was unable to locate anything authentic which would tell us as to who originated the ruku', and in what period. However, this much is almost certain that the purpose of this sign is to identify an average portion of verses which could be recited in one raka'ah. It is called 'ruku" so that ruku' (bending position) could be made in salah after reaching this point. There are 540 ruku'at in the whole Qur'an. So, by reciting one ruku' in every raha'ah of tarawih, it is possible to complete the recital of the entire Qur'an on the night of the twenty seventh (Fatawa 'Alamgiryah, Fasl al-Tarawih, 1/94).
Source:
http://www.as-sidq.org/mariful.html
Rukus are indicated by the Arabic letter 'Ain' and a number on it.
The Arabic style Qur'ans (printed in Syria, Saudi Arabia, Madinah etc) have hizbs in place of ruku's, and they are not necessarily on the same ayaat as the ruku's.
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