Ibn Rustah (in Persian: احمد ابن رسته اصفهانی - Aḥmad ebn Roste Eṣfahānī) was a 10th century Persian explorer and geographer born in Rosta district, Isfahan, Persia
His impression of the Rus' is very favourable (especially compared to the relation of Ibn Fadlan):
"They carry clean clothes and the men adorn themselves with bracelets and gold. They treat their slaves well and also they carry exquisite clothes, because they put great effort in trade. They have many towns. They have a most friendly attitude towards foreigners and strangers who seek refuge."
Of ancient Croatia he wrote in the chronicle Al-Djarmi:
"Their ruler is crowned … He dwells in the midst of the Slavs … He bears the title of 'ruler of rulers' and is called 'sacred king'. He is more powerful than the zupan (viceroy), who is his deputy … His capital is called Drzvab where is held a fair of three days every month."
About a certain king of the Caucasus Ibn Rustah wrote:
"He prayed on Fridays with the Muslims, on Saturdays with the Jews and on Sundays with the Christians. 'Since each religion claims that it is the only true one and that the others are invalid', the king explained, 'I have decided to hedge my bets.'"
He also travelled extensively in Arabia and is one of the early Persian explorers to describe the city of Sana'a. In his Book of Precious Records, he writes:
"It is the city of Yemen — there not being found in the highland or the Tihama or the Hijaz a city greater, more populous or more prosperous, of more noble origin or more delicious food than it. … San'a is a populous city with fine dwellings, some above others, but most of them are decorated with plaster, burned bricks, and dressed stones."

Literature

Ibn Rustah, Encyclopaedia Iranica, New York 2003.
Ibn Rustah, Kitāb al-A'lāk an-Nafīsa, ed. M. J. De Goeje, Bibliotheca Geographorum Arabicorum [BGA], Leiden, E. J. Brill, 1892