Despite research that was done, and the availability of the Wittgenstein-Sraffa correspondence, there seems to be no evidence for a specific philosophical influence on Sraffa on Wittgenstein. The short essay "Sraffa’s Impact on Wittgenstein" (Matthias Unterhuber, 2013) summarizes the currently available evidence on the subject.
The most famous story regarding Sraffa and Wittgenstein, concerning a certain gesture made by Sraffa, while riding on a train with Wittgenstein, was told by Norman Malcolm:
Wittgenstein was insisting that a proposition and that which it describes must have the same 'logical form', the same 'logical multiplicity'. Sraffa made a gesture, familiar to Neapolitans as meaning something like disgust or contempt, of brushing the underneath of his chin with an outward sweep of the finger-tips of one hand. And he asked: 'What is the logical form of that?'
Sraffa's gesture is said to have made a profound impression on Wittgenstein. Still, the details of this incident, and its interpretation, are in much dispute.
Sraffa was not a philosopher, but an economist. He didn't publish anything on philosophy, or on Wittgenstein.
The most plausible conclusion seems to me, that Sraffa influenced Wittgenstein by being for him a model of an intelligent person without philosophical training. Wittgenstein came to appreciate and to take into consideration Sraffa's raw pre-philosophical intuitions. Although there is no known direct link between Sraffa and any specific assertion or argument in Wittgenstein's texts.