It was St Anthony of Padua. According to The Little Flowers of St Francis, St Anthony tried to preach to heretics in Rimini, but they refused to listen. So the Saint went to the shore where a river emptied into the sea, and he said, "Listen to the word of God, O ye fishes of the sea and of the river, seeing that the faithless heretics refuse to do so." At his words, a multitude of fish approached the bank, with their heads out of the water, as if listening, while he preached to them.
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