Musannaf Ibn Abi Shaybah

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."