George tells Lennie not to drink the water because he's drinking too fast, it's dirty, and might make him sick.
His huge companion dropped his blankets and flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse. The small man stepped nervously beside him.
"Lennie!" he said sharply. "Lennie, for God' sakes don't drink so much."
Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. "Lennie. You gonna be sick like you was last night."
Lennie dipped his whole head under, hat and all, and then he sat up on the bank and his hat dripped down on his blue coat and ran down his back. "That's good," he said. "You drink some, George. You take a good big drink." He smiled happily.
George unslung his bindle and dropped it gently on the bank. "I ain't sure it's good water," he said. "Looks kinda scummy."
Of Mice and Men