Washington failed the test conducted by Major League Baseball in July, the Web site said. He has been subject to increased testing since then and has passed every test, SI.com said.
The Rangers have decided to keep Washington as their manager, SI.com said. The 57-year-old manager apologized for his mistake and told team management it was a one-time transgression.
"I had a very weak moment," Washington said at a news conference Wednesday. "I did wrong and I take responsibility for that and I'm sorry."