A similar narration is reported from Abu Ishaq (may Allah have mercy on him), and in it he added: 'As-salamu alaykum wa rahmatullah, as-salamu alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah, peace be upon you and the mercy of Allah). I also saw Abu Bakr and Umar (may Allah be pleased with them both) doing the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2976