Hi Ihab,
as Jigar mentioned, according to the PMBOK Guide 6th ed., it is the project charter that specifies who signs off on the project, together with the project approval requirements.
This is why the project charter is needed as an input to the Close Project or Phase process, as specified in Sect. 4.7.1, which also refers back to Sect. 4.1.3.1 for further details on the main elements specified in the project charter.
So, while formal acceptance of the deliverables occurs during the Validate Scope process, and Close Project/Phase confirms acceptance of the deliverables - as Devin detailed -, signing off on the project is also needed as part of the Close Project/Phase.
Gabriella