Sighing invalidates the prayer, unless the person is sick and cannot control themselves from groaning and sighing; because their groaning in this case is like sneezing if it results in letters. Ibn Abbas, may Allah be pleased with him, said that the Prophet (peace be upon him) said: "Blowing in prayer is speech," as recorded in Musannaf Ibn Abi Shaybah 2: 67, and Musannaf Abdul Razzaq 2: 189, and its chain of narration is authentic. See: I'laa al-Sunan 5: 51. And from Abu Huraira, may Allah be pleased with him: he said, "Blowing in prayer is speech," in Musannaf Abdul Razzaq 2: 189. See: Fath Bab al-Inayah 1: 302.