I passed by the Messenger of Allah (ﷺ) who was praying. I saluted him and he returned it by making a sign.

The narrator said: I do not know but that he said: He made a sign with his finger. This is the version reported by Qutaybah.
Sunan Abi Dawud Book 2, Hadith 536
← Previous Hadith We used to salute during prayer and talk about our needs. I came to the Messenger of Allah (ﷺ) and found him praying. I saluted him, but he did not respond to me. I recalled what happened to me in the past and in the present.

When the Messenger of Allah (ﷺ) finished his prayer, he said to me: Allah, the Almighty, creates new command as He wishes, and Allah, the Exalted, has sent a fresh command that you must not talk during prayer. He then returned my salutation.

Sunan Abi Dawud Book 2, Hadith 535 Next Hadith → The prophet of Allah(ﷺ) sent me to Banu al-Mustaliq. When I returned to him, he was praying on his camel. I talked to him; he made a sign to me with his hand like this. I again talked to him; he made a sign to me with his hand like this. I was hearing him reciting the Qur’an and he was making a sign with his head. When he finished his prayer, he said; what did you do about the mission for which I had sent you; nothing prevented me from talking to you except that I was praying.

Sunan Abi Dawud Book 2, Hadith 537