References for not receiving alms in exchange for Dhamma teaching

According to this comment : > I think that monks and laypeople are meant to depend on each other > (e.g. as described in Iti 107 ) however monks must not sell their > dhamma talks for food. **So there are monastic rules about food and > dhamma talks: they're separate transactions ... a *quid pro quo* is > forbidden** -- having given a dhamma talk (i.e. after his speaking) the > Buddha was no longer willing to receive food, because that would > appear as if he were receiving food in exchange for or as payment for > talking. Please provide references from the Vinaya and/or Patimokkha for "there are monastic rules about food and dhamma talks: they're separate transactions ... a *quid pro quo* is forbidden".