Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) relates: A man greeted the Prophet (peace and blessings be upon him) with salam while he was praying. The Prophet (peace and blessings be upon him) replied with a gesture. When he said the salam (ending the prayer), the Prophet (peace and blessings be upon him) said to that man: Previously we used to reply to the salam during the prayer, then we were forbidden from that.
Al-Bazzar recorded it; its chain contains Abdullah bin Salih, the scribe of al-Layth. Abd al-Malik bin Shu'ayb bin al-Layth declared him thiqah (trustworthy); he says: He is thiqah and ma'mun (reliable). But the Imams, namely Imam Ahmad and others, declared him weak.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2435
Hadith Gradingمحدثین:إسنادہ ضعیف، اس کی سند میں موجود راوی عبد اللہ بن صالح کاتب اللیث جمہور ائمہ کے نزدیک ضعیف ہے۔