BEIJING - Beijing's Niujie Mosque has many treasures: a five-century-old Koran, a gilded wooden plate engraved with an emperor' s order and foreign sheik tombs from hundreds of years ago.

But the most precious is a volume of Golestan, a Persian poem collection by renowned 13th century Iranian poet Saadi Shirazi,that is more than seven centuries old.

Chinese President Xi Jinping mentioned Saadi and his poem about Kashgar in west China's Xinjiang Uygur Autonomous Region in a signed article published in the Iran, the Iranian government's official daily newspaper, on Thursday, ahead of his state visit to the country.

In the fifth chapter of Golestan, Saadi wrote about meeting a young admirer in Kashgar.

Iranian leaders often visit the Niujie Mosque while on trips to Beijing to view the ancient book.

"Top Iranian leaders often appreciate the book for a long time," Zhang Lianci, head of the local Islamic association, said, pointing at the opened book behind a glass cover in the mosque's exhibition hall.

The writing is still clear, though the pages are broken on the fringe. Some Iranian officials offered to help restore the book, Zhang said.

In another exhibition hall of the mosque, a photo of then visiting Iranian President Ali Khamenei talking with a Chinese imam in 1989 is displayed high on the wall. Now he is Iran's supreme leader.

Among the long list of Iranian leaders who visited the mosque are former presidents Mohammad Khatami and Ali Akbar Hashemi Rafsanjani, Zhang said.

Saadi travelled extensively covering Iran, Iraq, Syria, Egypt, Ethiopia, India and Kashgar. He returned to his hometown Shiraz in southern Iran and wrote about his experience and thoughts from 30 years of traveling.

In Golestan, also known as Gulistan, meaning garden, Saadi wrote stories and used poems to underline the wisdom behind them. Golestan is also a textbook of Islamic education in the country.

Today, Saadi's cemetery has become a must-see for tourists in Shiraz along with the Persepolis ruins. As if paying homage to Golestan, the cemetery is designed with classical elements of a traditional Persian garden such as pools, geometrical patterns and delicately trimmed trees and flowers.

Zhang has little idea when the book was brought to the Niujie Mosque. But Chinese poet and writer Shen Wei wrote in an article comparing poems of Persia and China's Xinjiang that the handwritten Golestan was brought by an Islamic scholar from Persia in 1280 when Saadi was still alive.

First built in 996 AD, the 6,000-square-meter mosque is the oldest of its kind in Beijing. Comprised of ancient traditional buildings, it looks like a Chinese prince's palace except for the golden Arabic excerpts from the Koran on the walls.

Everyday, hundreds of people come to pray in the mosque on Niujie Street in downtown Beijing. More than 12,000 Muslims live around the street and are the largest Muslim community in Beijing, Zhang said. China has more than 20 million Muslims.

Tens of thousands of people would come to the mosque on important religious holidays, including many Iranians and Iranian diplomats, Zhang added.

Lianna Dasnimmapnazeri, a Malaysian student who studies Chinese at the Beijing Foreign Studies University, comes to pray in the mosque' s women section every month.

"I like the unique Chinese style. Praying is as solemn as in my hometown," she said.