Adding ‘wa Maghfiratuh’ in the response to the Salaam greeting
2-11-2010 | IslamWeb
Question:
As Salaam Alaikum Respected Scholar, Is it permissible to add supplements on greetings? When one greets by saying "Salaamu Alaikum wa Rahmatullahi wa Barakaatu" and the other replies the same adding "...wa Maghfiratu", so it this practice alright in Shari'ah? I have also read that Sahaaba use to add up to 30 words in greetings, is this true about them? Jazaak ALLAHu Khair.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
The jurists 
differed in opinion on the permissibility of adding more phrases to ‘wa Barakaatuh’ when responding to the Salaam greeting; Ibn Hajar, Ash-Shawkaani and Al-Mubaarak Poori are of the view that it is permissible. Their evidence is the saying of Allaah (which means): {When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally..}[Quran 4:86] Ash-Shawkaani 
said in his interpretation of the Quran: “What is meant by Allaah’s statement {…greet in return with what is better than it,} is to add more words in response to the expression that is said by the one who started the greeting. So, if the one who started the greeting says “'Assalaamu 'Alaykum” then the one who answers may say: “wa ‘Alaykum Assalaam wa Rahmatu Allaah”, and if the one who starts the greeting adds a word, then the one who answers the greeting may add one word or more to the sentence that is said by the one who started the greeting, like Wa Barakaatuh (blessings), Wa Mardhaatuh (pleasure), Wa Tahiyaatuh (greetings).”
Furthermore, Ibn Hajar cited a number of Ahaadeeth among which is the following:
Al-Bukhari reported in 'Al-Adab Al-Mufrad' from ‘Amr Ibn Shu'ayb from Saalim, the freed slave of Ibn ‘Umar 
that he said: “Ibn ‘Umar used to add more phrases when he responds to the Salaam greeting, so I came to him once and I said to him: ‘Assalaamu ‘Alaykum’ and he said: ‘Assalaamu ‘Alaykum wa Rahmatu Allaah’, then I came to him another time and I added ‘wa Barakaatuh’, so he responded by adding: … ‘Wa Tayyibi Salawaatih (special blessings)’. Also, Zayd Ibn Thaabit reported that Ibn ‘Umar wrote to Mu’aawiyah : “Assalaamu ‘Alaykum O Ameer Al-Mu’mineen (Leader of the Believers) wa Rahmatu Allaah wa Barakaatuhu wa Maghfiratuhu (forgiveness) wa Tayyibi Salawaatih.”
Moreover, there are other Ahaadeeth that are attributed to the Prophet about the permissibility of the addition to the Salaam greeting. Ibn As-Sunni reported with a weak chain of narrators on the authority of Anas who said: “A man used to pass by and says: 'Assalaamu 'Alayka ya Rasoola Allaah, and then the Prophet would say to him: ''Wa ‘Alayka Assalaamu wa Rahmatu Allaahi wa Barakaatuhu wa Maghfiratuhu wa Ridhwaanuh.”
Besides, Al-Bayhaqi reported in 'Shu’ab Al-'Eemaan' with a weak chain of narrators a Hadeeth on the authority of Zayd Ibn Arqam who said: "When the Prophet would say Salaam to us, we used to say: ''wa ‘Alayka Assalaamu wa Rahmatu Allaah wa Barakaatuhu wa Maghfiratuh.”
Ibn Hajar said: “If these weak narrations are combined together, they would strengthen the permissibility of the addition to the greeting after 'wa Barakaatuh'.”
In addition to this, Al-Bukhari reported in 'At-Taareekh' the Hadeeth reported by Al-Bayhaqi from Zayd Ibn Arqam and Al-Albaani classified it as Saheeh [authentic] in 'Silsilat Al-Ahaadeeth As-Saheehah'.
As regards the last thing that you mentioned in the question, then we are not aware of any evidence that proves it to be attributed to the Companions 
.
Allaah Knows best.