When is "and I bear witness..." said in responding to the adhan?

Question:
After the two testimonies, is it reported that one says: "And I bear witness that there is no god but Allah, I am pleased with Allah as my Lord..."?

Answer:
This is said at the two testimonies: "Whoever says when the mu'adhdhin says: I bear witness that there is no god but Allah... I am pleased with Allah as my Lord, with Islam as my religion, and with Muhammad as my Messenger; his sin will be forgiven." This is from the hadith of Sa'd ibn Abi Waqqas.
Q: May Allah reward you, is the narration: "And I bear witness that there is no god but Allah"?
The Shaykh: The meaning is the same, whether one says "I" or does not say "I," because the narrations differ.
Q: Is it authentic in terms of the narration?
The Shaykh: What appears is that it is authentic. If one says "I bear witness that there is no god but Allah," or says "And I bear witness that there is no god but Allah," he has said the same thing.