It is narrated from Sayyiduna Suhaib (may Allah be pleased with him) that I passed by the Messenger of Allah (peace and blessings be upon him) while he was praying. I greeted him with salam, and he replied with a gesture. The narrator of the hadith, Ibn Umar (may Allah be pleased with them both), says: To my knowledge, Sayyiduna Suhaib (may Allah be pleased with him) said: He (peace and blessings be upon him) replied with a gesture of his finger.
Hadith Referenceالسنن المأثورة / باب ما جاء في صلاة الخوف / 61
Hadith Takhrijسنن ابی داؤد، الصلاة، باب رد السلام فی الصلاة، رقم: 925؛ سنن ترمذی، الصلاة، باب ماجاء فی الاشارة فی الصلاة، رقم: 367؛ سنن نسائی، السهو، باب رد السلام بالاشارة فی الصلاة، رقم: 1186۔