In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Question: What is the ruling regarding a man who says, “Had it not been that I would make people grieve, I would have informed every person about the day in which he will die”; and what is your advice to the one who believes what this man says?

Answer: The one who believes what this man says has indeed disbelieved in that which has been revealed to Muhammad [sallal laahu alayhi Wa sallam] (ref 1), because he has believed that which nullifies the statement of Allaah: [قُل لَّا يَعۡلَمُ مَن فِى ٱلسَّمَـٰوَٲتِ وَٱلۡأَرۡضِ ٱلۡغَيۡبَ إِلَّا ٱللَّهُ‌ – Say (O Muhammad): None in the heavens and the earth knows the Ghaib (unseen) except Allaah]. [Surah An-Naml. Aayah 65]

It is well known that the [time, day and place] of death of a person is an affair of the unseen which none knows except Allaah [The Mighty and Majestic]; therefore if one claims to possess knowledge of this, then indeed he is a liar. The man who claims that he knows when people will die is no doubt a liar, and the one who believes what he says is a disbeliever (ref 2) due to the statement of Allaah: [قُل لَّا يَعۡلَمُ مَن فِى ٱلسَّمَـٰوَٲتِ وَٱلۡأَرۡضِ ٱلۡغَيۡبَ إِلَّا ٱللَّهُ‌ – Say (O Muhammad): None in the heavens and the earth knows the Ghaib (unseen) except Allaah]. [Surah An-Naml. Aayah 65]

And my advice to the one who believes what this man says is that he repents to Allaah and then holds as his belief that this man is a liar and is [submerged] in myths [illusion, superstitions]; and it is not permissible to believe what he claims regarding knowledge of the unseen. [Fataawaa Fadheelah Ash-Shaikh Al-Allaamah Muhammad Bin Saaleh Al-Uthaymeen Fil Aqeedah. Vol 1-2. Page 109. Slightly paraphrased]

Refs 1 and 2: Refer to articles by Shaikh Uthaymeen [rahimahullaah] regarding Takfeer of an individual [i.e. to pass the judgement against a person that he or she has disbelieved]:

http://www.spubs.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090003&articlePages=1

http://www.spubs.com/sps/sp.cfm?subsecID=MNJ09&articleID=MNJ090001&articlePages=1