There are numerous citation systems. For scientific works, generally, the IEEE-citation system is utilized. Each quote must be supported by a precise reference source.
In the text – reference number: as shown by Brown , ; as mentioned earlier , –, ; Smith  and Brown and Jones ; Wood et al. 
In addition to the correct citation system, also the bibliography plays an important role. It is located at the end of your thesis. Commonly, it is ordered alphabetically by the authors’ last names.
In the bibliography:
For further information concerning direct and indirect citation can be found in:
ThesisGuidelines by Prof. Dr. Prechelt.