It is narrated from Ata' (may Allah have mercy on him) that, in explaining this saying of Allah, the Mighty and Majestic: "The mercy of Allah and His blessings be upon you, O people of the house" [Surah Hud: 73] , he said: I was with Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) when a man came and greeted him with salam, and I replied: "Wa 'alaykum al-salam wa rahmatullahi wa barakatuhu wa maghfiratuhu (and upon you be peace, the mercy of Allah, His blessings and His forgiveness)." Ibn Abbas (may Allah be pleased with them both) (correcting me) said: "Stop where the angels stopped (that is, in returning the salam do not go beyond 'wa barakatuhu', for these are the words of the angels mentioned in the Qur'an)." This is a gharib (unusual) sahih (authentic) hadith narrated from Imam al-Thawri; to my knowledge it has been written only with this chain, and the two Shaykhs (Bukhari and Muslim) did not record it.