(a) Imran bin Husayn (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) gave the salam after leading three rak'ahs of the Asr prayer, and stood up and went into the room. Dhu al-Yadayn (may Allah be pleased with him) stood up and said: O Messenger of Allah, has the prayer been shortened? So he (peace and blessings be upon him) came out in a state of anger and led the rak'ah that had been left, then performed the two prostrations of forgetfulness and gave the salam.
(b) Imam Muslim (may Allah have mercy on him) recorded this hadith in his Sahih from Ishaq bin Ibrahim (may Allah have mercy on him), but in it he said: He (peace and blessings be upon him) led the rak'ah that had been left, after which he gave the salam and performed two prostrations, then gave the salam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3895