By 661, the Rashidun Caliphate spread from Libya-Tunisia border across Libya and Egypt, all of the Middle East, except for western Turkey, and small parts of Turkmenistan, Afghanistan, and Pakistan. The above picture is rather representative.
Asia and Africa
None. There were no Muslim armies in 661 BCE. Islam, as a religion based on Qur'anic Revelation would not come into existence until 610 CE, over 1200 years later. If the question intended to ask by 661 CE, the Muslims had conquered many modern countries by that point including parts of Libya, Egypt, Israel, Palestine, Jordan, Lebanon, Syria, Turkey, Georgia, Armenia, Azerbaijan, Iraq, Iran, Turkmenistan, Afghanistan, Kuwait, Saudi Arabia, Yemen, Oman, UAE, Qatar, and Bahrain. See the Related Link for a map.
The Muslims had conquered many modern countries by that point including parts of Libya, Egypt, Israel, Palestine, Jordan, Lebanon, Syria, Turkey, Georgia, Armenia, Azerbaijan, Iraq, Iran, Turkmenistan, Afghanistan, Kuwait, Saudi Arabia, Yemen, Oman, UAE, Qatar, and Bahrain. See the Related Link for a map.
the river that is completely within the Muslim lands of 661 is the Tigris River.
"Golden Age" is undefined. I would consider the golden age of the Muslim Civ to be the Abbasids c. AD 800. If that's true the answer is Baghdad. The center of the Muslim Civ originally was Mecca (630-661) and then Damascus (661-750).
Damascus became the capital of the Muslim world under the leadership of the Umayyad dynasty. The Umayyad dynasty ruled the Muslim world from 661 through 750.
110.1667
661 is prime.
3√661 ≈ 8.711
661 x 1
The river that is completely within the Muslim lands of 661 AD is the Euphrates River. During this period, the Umayyad Caliphate, established after the Rashidun Caliphate, controlled significant territories that included regions around the Euphrates, particularly in present-day Iraq and parts of Syria. This river was vital for agriculture and trade in the Islamic Empire, contributing to its economic and cultural development.
661 x 36 = 23796