It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) performed two prostrations after concluding the prayer with salam and after having spoken.
Against the statement of those who claim that if a person finishes the prayer and, having been forgetful in his prayer, speaks after the salutation out of forgetfulness, then he does not perform the two prostrations of forgetfulness; and this statement is contrary to what is established from the Sunnah of the Prophet, peace and blessings be upon him.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب السهو فى الصلاة / 1058