In the absence of any reliable document, from which Einstein’s ideas on relativity prior to 1905 could uniquely be drawn, we are forced to make assumptions. If many historians (J. Norton, O. Darrigol, …) have privileged an electromagnetic origin, others like A. Miller have emphasized the role of light quanta. I’ll come back on some evidences that the March quanta have played a role in the June and September 1905 relativity papers and reciprocally that the relativity principle may have influenced Einstein’s reflexions on free moving quanta. From both a scientific and an historical perspective, the association of quanta and relativity is indeed natural if one has in mind Planck’s work on the black body radiation and Poincaré’s contribution “Lorentz theory and the reaction principle” to the Lorentz Jubilee, which both occurred in mid-December 1900. As well known, the former introduced fixed energy quanta and the latter (quoted by Einstein in 1906) opened the way to a dynamical and relativistic (at first order in v/c) account of the electromagnetic field behaviour. But unknown, Poincaré’s paper also exhibited at this order the same transformation law for the energy and the frequency of an electromagnetic plane wave of finite length. If Einstein remarked this before 1906 (which is not proved), it may have encouraged his thinking of moving quanta. We would then meet the point of view expressed by R. Penrose in his preface of J. Stachel’s book Einstein’s miraculous year, that “it is virtually inconceivable that [Einstein] would have put forward two papers [March/June] in the same year which depended upon hypothetical views [particle/wave] of Nature that he felt were in contradiction with each other”. March, June and September 1905 Einstein's papers would constitute a remarkable synthesis, in a fully original and new epistemological view, of Planck's 1900 work on the interaction between matter and radiation and Poincaré’s 1900 paper on the relativistic dynamics of charges and the electromagnetic field.