The official RFC which defines this specification could be found here:
http://tools.ietf.org/html/rfc4021#section-2.1.2 (look at paragraph 2.1.2. and the following)
2.1.2. Header Field: From
Description: Mailbox of message author [...] Related information: Specifies the author(s) of the message; that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message. Defined as standard by RFC 822.
2.1.3. Header Field: Sender
Description: Mailbox of message sender [...] Related information: Specifies the mailbox of the agent responsible for the actual transmission of the message. Defined as standard by RFC 822.
2.1.22. Header Field: Return-Path
Description: Message return path [...] Related information: Return path for message response diagnostics. See also RFC 2821 [17]. Defined as standard by RFC 822.