Ten.....Twenty.....Thirty.

It is recommended for the one offering greetings to say: `As-Salamu Alaikum wa Rahmatullahi wa Barakatuhu'. The reply is `Wa `Alaikum us-Salamu wa Rahmatullahi wa Barakatuhu.'

`Imran bin Husain (May Allah be pleased with them) reported: A man came to the Prophet (sallallaahu ’alayhi wa sallam) and said: "As-Salamu `Alaikum (may you be safe from evil). Messenger of Allah (sallallaahu ’alayhi wa sallam) responded to his greeting and the man sat down. The Prophet (sallallaahu ’alayhi wa sallam) said, "Ten (meaning the man had earned the merit of ten good acts).'' Another one came and said: "As-Salamu `Alaikum wa Rahmatullah (may you be safe from evil, and Mercy of Allah be upon you).'' Messenger of Allah (sallallaahu ’alayhi wa sallam) responded to his greeting and the man sat down. Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "Twenty.'' A third one came and said: "As-Salamu `Alaikum wa Rahmatullahi wa Barakatuhu (may you be safe from evil, and the Mercy of Allah and His Blessings be upon you).'' Messenger of Allah (sallallaahu ’alayhi wa sallam) responded to his greeting and the man sat down. Messenger of Allah (sallallaahu ’alayhi wa sallam) said, "Thirty.''
[Abu Dawud and At-Tirmidhi].

Commentary: This Hadith shows that we can earn ten-fold good rewards by greeting a person in the Islamic way. There will be a further ten-fold addition to it if we say, "As-Salamu `Alaikum wa Rahmatullah". may you be safe from evil, and the Mercy of Allah be upon you). And if we say, "As-Salamu `Alaikum wa Rahmatullahi wa Barakatuhu'' may you be safe from evil, and the Mercy of Allah and His Blessings be upon you), thirty-fold good reward comes to us. But Ahadith are silent on increasing more words to Salam. So this much will suffice.
[Imam An-Nawawi's Riyad-us-Saliheen. Book 5: The Book of Greetings, Chapter 132: Words to be Used to Offer Greetings, number 851.]