It is narrated from Sayyiduna Samurah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When it is the time of the salam in the middle of the prayer or at the time of its conclusion, then before the salam say: All verbal worship, all pure words, all prayers (acts of bodily worship), peace and dominion belong to Allah alone. Then send salam upon the Prophet (peace and blessings be upon him), then send salam upon your near relatives and upon yourselves."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6989