"I want you to write, 'I must not tell lies'," she told him softly.
"How many times?" Harry asked, with a creditable imitation of politeness.
"Oh, as long as it takes for the message to sink in," said Umbridge sweetly. "Off you go." (OP13)
When he was serving detention, Dolores Umbridge forced Harry to write this sentence using her own magical quill. The words were instantly cut into the back of his hand and the words appeared on the paper with his own blood as the ink.
Rowling noted in her essay about Umbridge on Pottermore:
Dolores is the only person, other than Lord Voldemort, to leave a permanent physical scar on Harry, having forced him to cut the words 'I must not tell lies' on the back of his hand during detention. (WW)