(a) Nafi' narrates: I heard Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) say that the Messenger of Allah (peace and blessings be upon him) went to Quba'. The Ansar learned of his (peace and blessings be upon him) coming, so they came and began greeting him (peace and blessings be upon him) with salam (while he (peace and blessings be upon him) was in prayer). He says: I asked Sayyiduna Bilal or Sayyiduna Suhayb (may Allah be pleased with them both): How did you see the Messenger of Allah (peace and blessings be upon him) reply to the greeting in prayer? He said: He used to gesture with his hand.
Abu Isa al-Tirmidhi said: Both hadiths are sahih (authentic) in my view, and Ibn Umar narrated it from both Bilal and Suhayb.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3403