You can find an anthology of Luther's works called "What Luther says" and under "death" he writes "We Christians, however, who have been redeemed through the precious blood of God's Son should by practice become accustomed in faith to despise death and to look upon it as a deep, sound, sweet sleep, to consider the coffin as nothing but our Lord Jesus' bosom or Paradise, the grave as nothing but a soft bed of ease and rest. And indeed, this is just what it is before God; for He says 'Our friend Lazarus sleepeth' [John 11:11], 'The maid is not dead, but sleepeth' [Matt 9:24]"
However, I don't think it is correct to shove Luther's thought into this box and say "aha, this nicely kept doctrine is what he believes"
Here are some more quotes of Luther on death
"We do say: There is a difference between going to sleep and dying, but for Christians, there is in truth, no difference"
"The Fathers called churchyards coemeteria (cemeteries) that is, places in which one sleeps, sleeping quarters, where Christians are buried, and I wish they were still so called. So Isaiah says: 'they shall rest in their beds (57:2). To them the grave is not a tomb, but as it were, a bed in which they sleep until the time comes when they are to be awakened. (Isaiah 26:19)"
"Churchyards are called bedrooms and beds. These places of burial are held in peculiar honor and are called sleeping places (cemeteries). These names are a very fine testimony, whereby we confess by the manner of burying the dead that they will surely rise on Judgment Day. For the name of the burial place indicates that we carry the dead out in order to put them into the earth as into a bed of downy feathers. Moreover, the Holy Spirit calls dying and being buried lying down to sleep. For Christ says of Jairus' daughter, who had died 'give place, for the maid is not dead, but sleepeth," and He says to her: "Damsel, I say unto thee, arise" (Mark 5:41), that is, wake up, and promptly the damsel arose, just as if she have been lying in a deep sleep. How could we be willing, without this comfort, to look forward to so much adversity in this world? For Christians then, the churchyard is to be a sleeping place, for so God Himself calls it. The entire Holy Scripture, the Patriarchs, the prophets, and the entire Christian Church so call it"
"Since death is called a sleep, we know that we shall not remain in it, but we shall awake and live again, and the time during which we sleep cannot be long. It will seem as if we had just dropped off, so that we shall rebuke ourselves for having been appalled and frightened at so fine a sleep in the hour of death. And so from grave and corruption we shall in a moment go to meet our Lord and Savor Christ in the clouds. We shall be alive, entirely sound and vigorous, with a clean, bright, transfigured body. We should, therefore, with all confidence and joy commit and commend our soul, body, and life to Christ, as to our faithful Savior and Redeemer, even as we must without all care, commit our life to Him in bodily sleep and rest, certain that we shall not lose it, as it seems, but kept safely and well in His hand, it will be sustained and restored to us."
Perhaps it is better to investigate his position more closely and decide for yourself what he means.