Bispectrum from a time-derivative cubic scalar interaction

For a massless scalar in de Sitter spacetime with interaction $S_{\mathrm{int}}=(\lambda /3!)\int dt{d}^{3}x{a}^3{\dot{\varphi }}^2\varphi$, the tree-level late-time primordial bispectrum isIt scales as $B(\mu k_1,\mu k_2,\mu k_3)={\mu }^{-6}B(k_1,k_2,k_3)$.