The rule: When a person omits an obligatory act in his prayer, he prostrates for forgetfulness before the closing salam; its example and evidence.
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The Shaykh : As for omission, if a person omits something from the obligatory acts, such as the first tashahhud, and he stands up for it while forgetting, then if he stands up, he does not go back; he continues. However, it is obligatory for him to prostrate for forgetfulness. Is his prostration after or before the salam?
The student: Before the salam.
The Shaykh : Before the salam, because it is established from the Prophet (peace and blessings be upon him) that he stood up from the two rak'ahs without sitting, and when he completed his prayer, he prostrated two prostrations before he said the salam. The wisdom behind this is apparent: when an obligatory act is missing from the prayer, it needs to be completed, so it is wise to complete the prayer before finishing it.
The student: Before the salam.
The Shaykh : Before the salam, because it is established from the Prophet (peace and blessings be upon him) that he stood up from the two rak'ahs without sitting, and when he completed his prayer, he prostrated two prostrations before he said the salam. The wisdom behind this is apparent: when an obligatory act is missing from the prayer, it needs to be completed, so it is wise to complete the prayer before finishing it.