Respond three times to your brother when he sneezes, and if he sneezes more often, he has a cold in his head.
Sunan Abi Dawud Book 43, Hadith 262
← Previous Hadith When one of you sneezes, he should say: "Praise be to Allah in every circumstance," and his brother or his companion should say: "May Allah have mercy on you!" And he should then reply: "May Allah guide you and set right your affairs."

Sunan Abi Dawud Book 43, Hadith 261 Next Hadith → I know him that he traced this tradition back to the prophet (May peace be upon him).

Abu Dawud said: Abu Nu’aim transmitted it from Musa b. Qais, from Muhammad b. Ajlan, from Sa’id, on the authority of Abu Hurairah, from the prophet (May peace be upon him).

Sunan Abi Dawud Book 43, Hadith 263