Kolb's reflective cycle can found on pages 47-54 of this ebook:
Kolb, D.A. (2015) Experiential learning: experience as the source of learning and development. 2nd edn. New Jersey: Pearson Education.
Kolb originally published the model in 1984. He has since updated that book and re-published in this second edition of the same book. The second edition includes Kolb's commentary and thoughts on some of the key criticisms of the reflective cycle.
If you are referring specifically to the diagram, then your in-text citation would be (Kolb, 2015, p. 48) or Kolb (2015, p. 48) depending on the structure of your sentence.
