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Retail sales dipped 0.9 percent in July, the first sag in four months, the Commerce Department reported Tuesday. Sales fell to a seasonally adjusted $107.8 billion, after a 0.5 percent rise in June.

Echoing a 3.9 percent drop in general merchandise (including department store) sales, which was the greatest decline since July 1976, durable goods slumped 1.4 percent and nondurable goods, 0.6 percent.

This, the first decline since a 1.5 percent drop in March, supports other recent evidence that the economy, which had been running a strong head of steam in 1984, is beginning to slow.

The growth in consumer spending had been a key force driving the recovery from the 1981-82 recession.

Some analysts blamed cooler than normal weather in many parts of the country for the decline in retail sales. This would have depressed sales of such seasonal items as air conditioners and summer clothing.