It is narrated from Hazrat Ibn Umar (may Allah be pleased with them both) that I asked Hazrat Bilal (may Allah be pleased with him): If people greeted the Prophet Muhammad (peace and blessings be upon him) with salam while he was in the state of prayer, how would the Prophet (peace and blessings be upon him) respond to them? He replied: He would indicate to them with his blessed hand.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 23886
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن فى المتابعات والشواهد من أجل هشام بن سعد