Part NCO has been suggested, but has insruction really been designated as non-commercial?
It is not Commercial Air Transport so have a look at:
NCO.GEN.135 Documents, manuals and information to be carried. That will cover Private and Instructional flights.
Also:
FCL.045 Obligation to carry and present documents
(a) A valid licence and a valid medical certificate shall always be carried by the pilot when exercising the privileges of the licence.
(b) The pilot shall also carry a personal identification document containing his/her photo.
(c) A pilot or a student pilot shall without undue delay present his/her flight time record for inspection upon request by an authorised representative of a competent authority.
(d) A student pilot shall carry on all solo cross-country flights evidence of the authorisation required by FCL.020(a).