Mishkat al-Masabih #953

Nafi' said that ‘Abdallah b. ‘Umar passed a man while he was praying and gave him a salutation to which the man uttered some words in reply. ‘Abdallah b. ‘Umar went back to him and said to him, “When one of you is greeted while he is engaged in prayer he must not speak, but give a sign with his hand.” Malik transmitted it.