Here it is: At the end of Act 2, in Scene 4, there is this dialogue:
Is't known who did this more than bloody deed?
Those that Macbeth hath slain.
Alas, the day!
What good could they pretend?
They were suborn'd:
Malcolm and Donalbain, the King's two sons,
Are stol'n away and fled, which puts upon them
Suspicion of the deed.
’Gainst nature still!
Thriftless ambition, that wilt ravin up
Thine own life's means! Then ’tis most like
The sovereignty will fall upon Macbeth.
Ross has his doubts because it doesn't make any sense. A murderer has to have a motive, and as he asks, "What good could they pretend?" What in the world could they gain by doing such a thing? (Weirder still, although he doesn't ask it, why would they kill the King and then go back to sleep!?). The answer, from Macduff, is that they were secretly ordered to do the killing by the King's sons Malcolm and Donaldbain. That explanation, too, makes no sense to Ross, for their father was loved by them and was all they had.
Leave it to the Macbeths to come up with a plan that nobody in his right mind would believe.
In Act. II sc.4 Ross is surprised to learn from Macduff that it was the grooms who had killed King Duncan. Ross doubts whether the grooms would have killed King Duncan because Macbeth has himself killed these servants.
Alas, the day!What good could they pretend?
Ross suggests that the grooms seem to have no purpose or motive for killing their king and that they stand to gain nothing by it. Although Ross does not state it explicitly, he suspects that Macbeth has actually destroyed vital evidence by killing the grooms.
Macduff clarifies that the two servants had only been hired by the sons of Duncan, Malcolm and Donalbain who are the prime suspects, to commit the murder and that the sons have now fled the country. Ross once again remarks that this theory also does not hold water because why should the sons kill the goose that lays the golden egg:
'Gainst nature still! Thriftless ambition, that will raven up Thine own lives' means!
He seems to imply that Malcolm and Donalbain have always been faithful and loyal sons and that they wouldn't commit such a heinous deed.