Sahih Muslim Book 4, Hadith Number 1491.

‘Abdullah b. ‘Abbas reported that he said to the Mu’adhdhin on a rainy day: When you have announced “I testify that there is no god but Allah; I testify that Muhammad is the Messenger of Allah,” do not say: “Come to the prayer,” but make this announcement: “Say prayer in your houses.” He (the narrator) said that the people disapproved of it. Ibn ‘Abbas said: Are you astonished at it? He (the Holy Prophet), who is better than I, did it. Jumu’a prayer is no doubt obligatory, but I do not like that I should (force you) to come out and walk in mud and slippery ground.