He gets messages on his mobile phone that ask him to send them on to a specific number of people
I also received an e-mail from a man whose name is Mustafa. He said that he saw the Prophet Muhammad (PBUH) (in a dream), who ordered him to send his salaam (greeting of peace) to people. Also he said that everyone who reads this message should forward it to people, then to wait four days. After four days he will be very happy or very sad. What is the ruling on this? May Allah reward you.
For a Muslim to remind his brothers of good things and acts of righteousness, such as saying tasbeeh, tahleel and takbeer, is something that is prescribed and encouraged, because the one who guides others to do good is like the one who does it, whether it is a verbal reminder or a message sent via mail or mobile phone. If he urges them to send the message on to other people, this is also good, and the good deed will spread further.
What is not allowed is connecting that to a specific number, or saying that the person will be sinning if he does not send it on, such as saying, “Send it to ten people, and if you do not do this then such and such will happen to you.” This is lying and falsehood, and obliging people to do something that sharee’ah does not oblige them to do. There is the fear that the one who does that may be exposed to the punishment of one who fabricates lies about Allaah, may He be exalted, and introduces into the religion things for which Allaah has not given permission.
An example of that is what you mention in your question, where the sender of the message said: “Everyone who reads this message should forward it to people, then wait four days. After four days he will be very happy, or else he will be very sad.” This is fabrication, lying and introducing innovations into the religion. See also question no. 31833.
Hence it is not permissible to distribute this message or to believe the promises or warnings contained in it; rather it should be destroyed and people should be warned against it and against those who send it.
And Allaah knows best.