An iPhone 6 Plus at an Apple store

Most of Apple's revenue comes from iPhone sales, and at least one firm is predicting the first year-over-year decline in sales for 2016.

Sales of Apple's flagship product are likely to drop up to 8 percent next year, said Andy Hargreaves and Evan Wingren of technology investment research firm Pacific Crest Securities. The massive number of consumers that upgraded to the iPhone 6 when it was released last September is "unrepeatable," they said in a note Sunday.