In response to a question, I have to answer.
A: Hello B, can I go?
B: Ask C first.
A: I have already asked from C.
Is this grammatical?
In British English:
Are both correct.
You could probably also say:
You would definitely not say:
as that is American English, not British English.
To go straight to the answer, I have already asked from C is ungrammatical, because in most contexts ask is complemented by an indirect object (C) and not a preposition phrase (from C) to refer to the person asked. It has to be I have already asked C.
A can answer:
But ask isn't followed by the preposition from. You can ask someone of something, i.e.,
But that is prohibitively formal for conversation.
Follow-up answer to ash's answer: Non native speakers frequently have a prepositional or conceptual usage pattern in their native language that makes them interchange "of" and "from."