Ibrahim and Hazrat Sha'bi state that two prostrations are obligatory for every mistake.

4551 - From Shu’bah, from Jabir, from al-Sha’bi, they said: For every forgetfulness, there are two prostrations.

Hazrat Thawban says that the Holy Prophet Muhammad (peace and blessings be upon him) said that for every forgetfulness, two prostrations are obligatory.

Hazrat Abu Farwah says that Hazrat Abdur Rahman bin Abi Laila led us in prayer. When he sat down, he (by mistake) moved to stand up and performed two prostrations of forgetfulness (sujood as-sahw).

Hazrat Yahya bin Saeed says that Hazrat Anas led us in two rak'ahs of Asr prayer during a journey. When he sat in the second rak'ah, he forgot to say salam and began to stand up. At this, the people said tasbih. When he sat down, he said salam and performed two prostrations of forgetfulness (sajda sahw).

Hazrat Abdul Aziz bin Suhaib says that Hazrat Anas bin Malik (may Allah be pleased with him) sat for Tashahhud in the third rak'ah, so the people behind him said tasbih. He stood up and completed the fourth rak'ah. When he finished with salam, he performed two prostrations of forgetfulness (sujood as-sahw). Then, turning towards the people, he said: "When you are in doubt, do like this."

Hazrat Abu Ja'far and Hazrat Ata say that forgetfulness (sahw) occurs in addition and omission.