The first observation of the rare decay$B^0_s to phipi^+pi^-$ and evidence for $B^0 to phipi^+pi^-$ are reported, using $pp$ collision data recorded by the LHCb detector at centre-of-mass energies $sqrt{s} = 7$ and 8~TeV, corresponding to an integrated luminosity of $3{mbox{,fb}^{-1}}$. The branching fractions in the $pi^+pi^-$ invariant mass range $400<m(pi^+pi^-)<1600{mathrm{,Mekern -0.1em V!/}c^2}$ are $[3.48pm 0.23pm 0.17pm 0.35]times 10^{-6}$ and $[1.82pm 0.25pm 0.41pm 0.14]times 10^{-7}$ for $B^0_s to phipi^+pi^-$ and $B^0 to phipi^+pi^-$ respectively, where the uncertainties are statistical, systematic and from the normalisation mode $B^0_s to phiphi $. A combined analysis of the $pi^+pi^-$ mass spectrum and the decay angles of the final-state particles identifies the exclusive decays $B^0_s to phi f_0(980) $, $B_s^0 to phi f_2(1270) $, and $B^0_s to phirho^0$ with branching fractions of $[1.12pm 0.16^{+0.09}_{-0.08}pm 0.11]times 10^{-6}$, $[0.61pm 0.13^{+0.12}_{-0.05}pm 0.06]times 10^{-6}$ and $[2.7pm 0.7pm 0.2pm 0.2]times 10^{-7}$, respectively.
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