Since the living know they will die. But the dead know nothing; they have no more recompense, for even the memory of them has died. Their loves, their hates, their jealousies have long since perished; and they have no more share till the end of time in all that goes on under the sun. Go, eat your bread in gladness, and drink your wine in joy; for your action was long ago approved by God. (Ecc 9:5-7; 1985 JPS Tanakh)
This is one of the most commonly cited biblical texts in favour of either soul-sleep or soul-death, a doctrine held by groups such as the Jehovah's Witnesses, Seventh Day Adventists, Christadelphians, and other groups, although this goes against the teaching of other texts that clearly explicate that the dead are conscious, such as 1 Pet 3:18-20, a key text in favour of the LDS doctrine of posthumous salvation (see a discussion here).
R.B.Y. Scott, in his commentary on Proverbs/Ecclesiastes, offered the following exegesis of vv.5-6:
With this unequivocal statement about death, cf. vs. 10 and Pss. vi 5 EV, lxxxviii 12 EV, cxv 17; Job xiv 10-12. Other OT writers speak of the "shades" of men as still possessing consciousness and memory, where they "dwell" in the gloomy, dusty cavern of Sheol beneath the earth (Num xvi 30-33; 1 Sam xxviii 8-14; Ps xcliii 3; Isa xiv 9-11, 15-17). Job ponders the possibility of resurrection (xiv 14-17, xix 25-27), and in the eschatology of late prophecy and apocalyptic writing resurrection if affirmed (Isa xxvi 19; Dan xii 2). (R.B.Y. Scott, Proverbs/Ecclesiastes [Anchor Bible; Garden City: Doubleday, 1965], 246)
Recommended Reading
Philip S. Johnston, Shades of Sheol: Death and the Afterlife in the Old Testament (IVP Academic, 2002)
Jeff Linday, Mormon Answers: Questions about the Dead (LDS treatment of the intermediate state in response)