Authentic Supplication Number 219.

`Imran bin Husayn said: A man came to the Prophet  and said: 'As-salamu `alaykum, so he replied then he sat down, and then the Prophet  said "Ten." then another came along and said 'As-salamu `alaykum wa rahmatullah, so he replied, then he sat down, and then he said: "Twenty." Then another came along and said: 'As-salamu `alaykum wa rahmatullah wa barakatuhu, so he replied, then he sat down, and then he said: "Thirty."

Reporters.
At-Tirmithi said it was comely. Al-Albani agreed, and said that so did Al-Bayhaqi, and Al-Hafith Ibn Hajar. Al-Albani included this hadeeth in The Authentic of Good Sayings as #156.