Sahih Bukhari, Chapter: Call to Prayers (Adhaan)

Once on a rainy muddy day, Ibn 'Abbas delivered a sermon in our presence and when the Mu'adhdhin pronounced the Adhan and said, "Haiya ala-s-sala(t) (come for the prayer)" Ibn 'Abbas ordered him to say 'Pray at your homes.' The people began to look at each other (surprisingly). Ibn 'Abbas said. "It was done by one who was much better than I (i.e. the Prophet or his Mu'adhdhin), and it is a license.'