In Welsh, many verbs are followed by specific prepositions.  A comprehensive list of verb-nouns followed by their correct prepositions may be found by clicking here.

In this instance, dweud is usually followed by wrth (when followed by a person) or am (when followed by the topic of conversation), e.g.

Mae Rhys wedi dweud wrth ei rieni. (Rhys has told his parents.)
Mae Rhys wedi dweud am y ddamwain. (Rhys has talked (lit. told) about the accident.)