Sahih Muslim Book 25, Hadith Number 5363.

Chapter : Disapproval of saying: “It is I” In response to one’s inquiry while seeking permission to get in the house.

Jabir b. Abdullah reported: I came to Allah’s Apostle (may peace be upon him) and called him (with a view to seeking permission), whereupon Allah’s Apostle (may peace be upon him) said: Who is it? I said: It is I. Thereupon he (the Holy Prophet) came out saying: It is I, it is I.