Mawla of the Messenger, Sayyiduna Thawban (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him and his family) said: For every forgetfulness, there are two prostrations after completing the prayer with salam.

It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him and his family) led them in prayer and he (peace and blessings be upon him and his family) forgot. Then when he (peace and blessings be upon him and his family) finished the prayer with salam, he performed two prostrations and then finished with salam again.

It is narrated from Sayyiduna Abdullah bin Ja'far (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: If a person has doubt in his prayer, then after saying salam he should perform two prostrations.