What suspicion does Banquo voice in "Macbeth"? What does he say that lets us know what he suspects?

At the beginning of Act 3, Banquo, in a brief soliloquy says, "Thou hast it now - King, Cawdor, Glamis, all, As the Weird Women promised; and I fear Thou play'dst most foully for't." He is saying that Macbeth (Thou) has seen all the witches' prophecies come true, but he thinks that Macbeth played foul and committed murder (play'dst most foully) to make the prophecies come about.  Even back in Act 2, sc. 1, before the killing of Duncan, Macbeth tells Banquo that he wants to talk with him sometime on the subject of the witches' prophecies and he goes on to hint to Banquo that listening to him, Macbeth, could be good in the long run for Banquo.  Banquo's response is that he'll listen to what Macbeth has to say as long as he can keep his conscience clear and show no disloyalty to the king.  Macbeth probably knew from that remark that Banquo would not go along with the murder or any knowledge of the murder.