In 1905 Einstein explained the origin of Planck spectrum by demanding that the quantization of energy actually belonged to radiation itself, not merely an artifact of how energy is distributed. He also insisted that this was a universal feature, common to all emission and absorption of radiation. He predicted the photoelectric law now known after him. In 1905 it was not a properly measured effect.
The hypothesis that radiation was universally emitted and absorbed in lumps was so radical and so contradictory to Maxwell theory that it was taken to be complete humbug. For several decades, even after the photoelectric effect was established in great detail there was pressure on Einstein to withdraw his ``spurious'' photon hypothesis.
The Nobel Prize in 1922 was given to him, not for correct physical explanation of the photoelectric effect but only for predicting the formula describing the phenomenon.