It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that none of you should say about something he does not know that "Allah knows," while what Allah knows is something else. Thus, he is including in Allah's knowledge something he does not know. This is a great sin in the sight of Allah. (That is, if someone says, "Allah knows, the truth is such," while in reality the truth is not such, then he is attributing to Allah's knowledge something which is not true in reality, so it should not be attributed to Allah.)