It is a part of the Prophetic Sunnah to remind the Muslim when dying with the word of Tawheed ( la ilaha illa Allah ), those who are present with him or one of them shall utter it; perhaps he says it, then it will be the last of his words in this world. His heart will be steadfast with this word until his soul ascends to its Creator. Then it will continue to be a light in his grave until the resurrection when he meets his Lord.