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