"A man came to the Prophet (ﷺ) and said: 'As-Salamu 'Alaykum (Peace be upon you).'" "So the Prophet (ﷺ) said: 'Ten.' Then another came and he said: 'As-Salamu 'Alaykum Wa Rahmatullah (Peace be upon you, and the mercy of Allah).' So the Prophet (ﷺ) said: 'Twenty.' Then another came and said: 'As-Salamu 'Alaykum Wa Rahmatullahi Wa Barakatuh (Peace be upon you, and the mercy of Allah, and His Blessings).' So the Prophet (ﷺ) said: 'Thirty.'"
Jami` at-Tirmidhi Book 42, Hadith 2
← Previous Hadith that the Messenger of Allah (ﷺ) said: "By the One in Whose Hand is my soul! You will not enter Paradise until you believe, and you will not believe until you love one another. Shall I inform you about a matter which if you do it, then you will love one another? Spread the Salam among each other."

Jami` at-Tirmidhi Book 42, Hadith 1 Next Hadith → "Abu Musa sought permission to enter upon 'Umar. He said: 'As-Salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: 'Once.' Then he was silent for some time. Then he said: 'As-Salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: Twice.' Then he was silent for some time. Then he said: 'As-salamu 'Alaykum (Peace be upon you). May I enter?' 'Umar said: 'Three times.' Then he (Abu Musa) left. 'Umar said to the gate-keeper: 'What did he do?' He replied: 'He left." He said: 'Bring him to me.' So when he came, 'Umar said to him: 'What is this that you have done?' He said: 'The Sunnah.' He said: 'The Sunnah? By Allah! You had better bring me proof or a witness to clarify this, or I will do this or that to you.'" He said "So he came to us while we were sitting with Ansar. He said: 'O People of the Ansar! Are you not the most knowledgeable people about the Ahadith of the Messenger of Allah (ﷺ)? Did the Messenger of Allah (ﷺ) not say: "Seeking permission is to be done three time. Either you are permitted, or other wise leave?" The people began joking. Abu Sa'eed said: "Then I raised my head toward him and said: "Whatever punishment you are afflicted with because of this, then I shall be your partner in it." So he went to 'Umar to inform him him about it, and 'Umar said: "I did not know about about this."

Jami` at-Tirmidhi Book 42, Hadith 3