The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: ‘And whoever forgets a prostration’ up to his statement: ‘And whoever speaks’, its explanation has already passed in the previous chapter, so there is no need to repeat it. Then he, may Allah سبحانه وتعالى have mercy on him, said: ‘Whoever speaks deliberately during the prayer with something that is not permissible to say during the prayer must repeat it. But whoever speaks out of forgetfulness should perform the two prostrations of forgetfulness (‘sajdatay al-sahw’) and is not required to repeat the prayer.’
Commentary #18