Still, the index was on course to rise 0.4 percent for the month of August.
The most influential movers on the index included Suncor Energy Inc, which fell 1.9 percent to C$35.73, and TransCanada Corp, which declined 1.2 percent to C$59.14.
US crude prices were down 2.2 percent to $45.34 a barrel, extending losses as data showed a bigger-than-expected increase in US stockpiles.
The energy group retreated 1 percent, while the materials group, which includes precious and base metals miners and fertilizer companies, lost 0.7 percent.
Barrick Gold Corp fell 2.1 percent to C$22.33, pressured by lower gold prices.
Gold slid to a two-month low after forecast-beating US jobs data stoked speculation that the Federal Reserve would move ahead with plans to raise interest rates. At 10:40 a.m. EDT (1440 GMT), the Toronto Stock Exchange's S&P/TSX composite index fell 50.83 points, or 0.35 percent, to 14,634.02.
Seven of the index's 10 main groups were lower.
National Bank of Canada reported a 6-percent increase in quarterly profit, benefiting from a recent acquisition that enabled it to beat market expectations.
Still, its shares fell 2.4 percent to C$46.45, while the overall financials group dipped 0.1 percent.
Canada's economy shrank in the second quarter in its worst showing in seven years, hurt by a drop in exports and a disruption to oil production caused by wildfires in northern Alberta, data from Statistics Canada showed.