I will find the source for this citation and post it in the near future. However, I have fifty books on Albert Einstein and his writings, so it may take some time. I believe the quotation is to be found in one of these three writings of Albert Einstein:
"On the Theory of the Generation and Absorption of Light", Analen der Physik, 20, 1996, pp. 199-206.
"On the Development of Our Views on the Nature and Constitution of Radiation", Lecture, Salzburg, Verhandlungen der Deutschen Physikalischen Gesellschaat, 11, 1909, pp. 482-500.
"Theory of Light Quanta and the Question of the Location of Electromagnetic Energy", Archives des sciences physiques et naturales", 29, 1919, pp. 525-528.
