What is the ruling on saying: "God knows," for example: "God knows I went to a friend"?

It is permissible to say "God knows" if the speaker is truthful. As for the liar, it is forbidden for him, and it may lead him to disbelief if he intends that God knows his lie. The jurists have differed regarding the necessity of expiation for one who says it falsely. Some have deemed it obligatory, while others have not. A third group elaborated, saying: if he intended by it an oath, then expiation is due upon him, otherwise not. A group also recommended expiation as a precaution.