Or HaChaim on Exodus 16:26/אור החיים על שמות ט״ז:כ״ו

1 ששת ימים תלקטהו, "During six days you will collect it, etc." This is meant as a warning not to go out on the Sabbath in order to collect manna; the meaning of the words לא יהיה בו, refers to the activity of collection. Moses had already announced that there would not be any manna when He said: "you will not find it in the field." This is the reason that G'd became so angry (verse 28) asking "how long will you refuse to observe My commandments?" He referred to the people violating a negative commandment. Where else do we find that G'd had forbidden the attempt to collect manna on the Sabbath if not in verse 25? If you were to assume that the prohibition was to be derived from verse 26, this is hardly possible seeing that our verse speaks only about what to do, not about what not to do. At best it would be considered as לאו הבא מכלל עשה, a negative commandment which is a corollary of a positive commandment (Pessachim 41). It would not be justifiable to refer to a violation of such a commandment with the words "you have refused, etc."