The National Association of Realtors said their Pending Home Sales Index slipped 5.6 percent in September to 101.6 from a downwardly revised August level of 107.6.
The index is 1.2 percent lower than the September 2012 reading of 102.8.
NAR Chief Economist Lawrence Yun said the index, which hit a six-year peak in May, was likely reflecting eroding affordability.
"Declining housing affordability conditions are likely responsible for the bulk of reduced contract activity," Yun said in a statement, noting it was the first time in 29 months that the index was below its 12-month prior level.
"This tells us to expect lower home sales for the fourth quarter, with a flat trend going into 2014," Yun said.
The Pending Home Sales Index, which reflects sales activity one and two months down the road, is a comparison to the monthly average for 2001, its first year, which was assigned a value of 100.
The NAR said the Pending Home Sales Index for the Northeast dropped 9.6 percent to 76.7 -- a level that is 6.4 percent lower than September 2012.
In the Midwest, the index, dropped 8.3 percent to 102.3, reaching a level 5.7 percent higher than September 2012.
The index fell 0.4 percent in the South to 116.2, which is 2 percent higher than 12 months prior. In the West, the index came to 97.3, down 9 percent from August and 9.8 percent lower than September 2012.