Following the Mu'adhdhin if he adds to the Takbir in forgetfulness

Question:
Honorable Shaykh, Salih says: If the mu'adhdhin adds to the takbir at the beginning of his adhan, or shortens it, out of forgetfulness, is it required for the one who hears him to follow him for the rest of the adhan?

Answer:
Yes, he should follow him, but he should not follow him in the addition; rather, he should follow him in the prescribed adhan. However, as long as it was out of forgetfulness, there is no blame on him and the adhan is valid. But if he adds to the adhan intentionally, then this is a reprehensible matter and a religious innovation (bid'ah), so it is not permissible. But if he does it out of forgetfulness, the adhan is valid, and the one who hears him follows him without the addition.