Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that in the beginning, we used to greet the Prophet (peace and blessings be upon him) with salam during prayer, and he would reply to us. But when we returned from (the land of) Najashi and greeted him with salam, he did not reply and said: "In fact, there is engagement in prayer."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 3884
Hadith Gradingحکم دارالسلام:حديث صحيح، خ: 1199، م: 538، وهذا إسناد ظاهرة الانقطاع، إبراهيم النخعي لم يسمع من ابن مسعود.