Using the production reactions and , the Dalitz plot distribution for the decay is studied with the WASA detector at COSY, based on a combined data sample of events. The Dalitz plot density is parametrised by a product of the P-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables Z and ϕ. For the first time, a deviation from pure P-wave phase space is observed with a significance of 4.1σ. The deviation is parametrised by a linear term , with α determined to be, consistent with the expectations of ρ-meson-type final-state interactions of the P-wave pion pairs.