They are contained in unit instructions, which go by different names (such as LATSI- local air traffic services instructions, MATS- manual of air traffic services etc.), and are unfortunately not available online, to my knowledge. The 'head of power' for such a rule is ICAO DOC 4444 10.2.1 "Air traffic control units shall issue such air traffic control clearances as are necessary to meet the objectives of collision prevention and the expedition and maintenance of an
orderly flow of air traffic."