Abdullah bin Umar (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) entered the mosque of Banu Amr bin Awf to pray in it, and Suhayb (may Allah be pleased with him) entered with him (peace and blessings be upon him). Then some men of the Ansar came to greet him (peace and blessings be upon him) with salam. Abdullah (may Allah be pleased with him) said: So I asked Suhayb (may Allah be pleased with him) what the Messenger of Allah (peace and blessings be upon him) used to do when he was greeted with salam during the prayer. He said: He (peace and blessings be upon him) used to gesture with his hand.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7260
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) came to the mosque of Quba' and was praying, and the people began to enter and greet him (peace and blessings be upon him) with salam. When Sayyiduna Suhayb (may Allah be pleased with him) came out, I asked him how the Messenger of Allah (peace and blessings be upon him) used to respond to their greeting. He said: "With his hand, like this," and he gestured with it.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7261
Abdullah bin Umar (may Allah be pleased with them both) narrated from Suhayb (may Allah be pleased with him) that he said: "I passed by the Messenger of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was praying, so I greeted him (peace and blessings be upon him) with salam, and he (peace and blessings be upon him) answered me with a gesture."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7262