Sayyiduna Imran bin Husayn (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings be upon him) said the salam after three rak'ahs of the Asr prayer, then stood and went into his apartment. Then al-Khirbaq - a man with long hands - stood up and said: O Messenger of Allah, have you shortened the prayer or have you forgotten? So he (peace and blessings be upon him) came out angry, dragging his garment, asked, and was told, "so he (peace and blessings be upon him) prayed the rak'ah that had been left out, then said the salam, then performed the two prostrations of forgetfulness, then said the salam."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15558