In Twelver Shia Islam, Uthman ibn Sa'id al-Asadi was the fourth and last of The Four Deputies appointed by the twelfth and final Imam, Muhammad al-Mahdi, while he was in the Minor Occultation. Unlike his predecessors he had not appointed a successor, and the Major Occultation began.
He was a companion of the 11th Shia Imam Hasan_al-Askari and was designated as the fourth and last of the The Four Deputies for a period of 3 years after the death of the third deputy Abul_Qasim_Husayn_ibn_Ruh_al-Nawbakhti. 6 days before his death, he received a letter from the twelfth and final Imam, Muhammad al-Mahdi in which he was notified that there is no deputy after him, a translation from Farsi to English of the letter is as follows:
"In the name of Allah, the Compassionate the Merciful
Oh Mohammad ibn Ali Samari, In the grievance of your death shall God grant your brothers a great reward. You will leave this world in 6 days. Settle your affairs and do not assign a successor for yourself. The time of the Major Occultation has arrived and I will not reappear until Allah grants me the permission to do so, and my reappearance will only be after a long time and the pitilessness of hearts and the overspread of injustice on the Earth. There will be those who will announce themselves as my deputy to my Shias. Be aware that if anyone before the emergence of the Sufyani and the loud announcement from the sky, makes any such claims (to be my deputy), then they (he/she) is a liar and a deceiver as all movements and influences are from none but Allah."
On the 6th day after receiving the letter, Abul Hasan Ali ibn Muhammad al-Samarri passed away (329 AH) and he was buried in Baghdad near the current grave of Sheikh Kulayni. Before his death, they asked him as to who the next deputy was and he responded: I dont have permission to introduce anyone.
Following his death, the Major Occultation began.