Shaykh Abu Jaafar, Mohammad bin Ya’qoub bin Is’haaq Al-Kulayni was born in the second half of the third century in the town of Kulayn, located thirty-eight kilometres from the city of Ray, south of Tehran. He died in the month of Shaaban in the year three hundred and twenty-nine. 

He was the leader of the scholars in Ray during his time. He was the most  trustworthy scholar in the field of Hadith narration. He left behind many invaluable books including the honourable book, Al-Kafi, a work to which he devoted twenty years of his life. 

Ibn Atheer comments on this Hadith narrated from the Messenger of Allah, 

who said, “Allah sends one man in every century to revive His religion and to renew His glorification and praise” by saying: 

The reviver of the religion of the Shia in the second century was Mohammad bin Ali Al-Baqir ; in the third century, it was Ali bin 

Musa Al-Redha; and in the fourth century, it was Mohammad bin Ya’qoub Al-Kulayni.

Shaykh Al-Najaashi praises him by saying, “He was the leader of the Shia in the city of Ray. He was the most accurate scholar in narrating Ahadith. He was an undisputed scholar who was accepted by all scholars.” 

Sayyid bin T’awoos praises him by saying, “The trustworthiness and the 

honour of Kulayni was agreed upon by all of the scholars.”