I can give you 750 examples.
No United States Constitution provision, federal statute or common-law principle explicitly permits, or prohibits signing statements. However, Article I, Section 7 (in the Presentment Clause) empowers the president to veto a law in its entirety, or to sign it.
Article II, Section 3 requires that the executive "take care that [those] laws be faithfully executed".
Signing statements do not appear to have legal force by themselves. As a practical matter, they may give notice of the way that the Executive intends to implement a law, which may make them more significant than the text of the law itself. They may also be included as part of legislative history in clarifying the intent of a law.