From: Anecdotes of the Pious Ones, #70 v. 1
Abu Ishaq-e Sabi is a famous 4th century scholar and writer. He was a state accountant at the court of Abbasid Caliph for a time and at the court of 'Izzud Dawla Bakhtyyar (from Ali-a Booya).
Abu Ishaq was Sabi who believed on the Oneness of Allah but they did not believe in the Prophethood.
'Izzud Dawla tried very hard to persuade him to become a Muslim, but he didn't convert.
Abu Ishaq used to fast during the month of Ramadhan respect to Muslim. And he had memorized a large portion of the Holy Qu'ran. He used to quote Qu'ran in his letters and in his writings. Abu Ishaq was a learned intellectual and poet and was a friend of Sayyed Radhi who was a genius of his own time.
Abu Ishaq died in 384 Hijri and Sayyed Radhi said the following lines about him: "Did you see who was carried the coffin? Did you see how the light of the gathering dimmed? A Mountain fell, and if it had fell in the ocean, it would create a storm there in. I didn't believe that dust will cover you for dust doesn't cover high rising mountains."
After that some narrow vision people blamed Sayyed who was from the family of the Prophet (pbuh) and would praise a Sabi man who denied religions.
Sayyed said: "I praised him for his knowledge, in fact I praised knowledge." 
 Wafyat al A'yaan ibn Khalakan v. 1 , p. 36, v.2 p. 365 title of Al Sabi