Or HaChaim on Numbers 27:6/אור החיים על במדבר כ״ז:ו׳

1 ויאמר ה׳ אל משה לאמור, G'd told Moses to say, etc. The reason G'd said לאמור was to make certain Moses would not think that when G'd told him that the daughters of Tzelofchod had a valid claim he was to keep this information to himself and need not reveal it. It is even possible that the word לאמור implies that Moses was not only to acknowledge the daughters' claim to them but that he was to tell the whole Jewish people about it. This is why the Torah did not write לאמור להן "to tell them." It is also possible that seeing the Torah writes ואל בני ישראל "and to the children of Israel you shall say" in verse 8 the Torah wanted to reveal that what G'd had told Moses prior to verse eight did not need to be mentioned except to the daughters of Tzelofchod themselves. The word לאמור in our verse then applied only to the daughters.