I'm not sure if the Bible ever calls laziness a sin, but it certainly admonishes against it, as in these verses:
Proverbs 6:6-11 Go to the ant, o sluggard, observe her ways and be wise, which having no chief, officer or ruler, prepares her food in the summer and gathers her provision in the harvest. How long will you lie down, o sluggard? When will you arise from your sleep? "A little sleep, a little slumber, a little folding of the hands to rest" -- Your poverty will come in like a vagabond and your need like an armed man.
Proverbs 10:26 Like vinegar to the teeth and smoke to the eyes, so is the lazy one to those who send him.
Proverbs 13:4 The soul of the sluggard craves and gets nothing, but the soul of the diligent is made fat.
Proverbs 19:15 Laziness casts into a deep sleep, and an idle man will suffer hunger.
Proverbs 21:25 The desire of the sluggard puts him to death, for his hands refuse to work;
Proverbs 28:19 He who tills his land will have plenty of food, but he who follows empty pursuits will have poverty in plenty.
Proverbs 31:10 An excellent wife who can find? For her worth is far above jewels.... Proverbs 31:27 She looks well to the ways of her household, and does not eat the bread of idleness.
2 Thes. 3:10-12 For even when we were with you, we used to give you this order: if anyone will not work, neither let him eat. For we hear that some among you are leading an undisciplined life, doing no work at all, but acting like busybodies. Now such persons we command and exhort in the Lord Jesus Christ to work in quiet fashion and eat their own bread.