Sahih Muslim Book 001, Hadith Number 0022.
Chapter : Relating to the Command for Belief In Allah and His Prophet and the laws of Islam and inviting (people to) them.
It is narrated on the authority of Ibn 'Abbas that a delegation of Abdul Qais came to the Messenger of Allah (may peace be upon him) and said: Messenger of Allah, verily ours is a tribe of Rabi'a and there stand between you and us the unbelievers of Mudar and we find no freedom to come to you except in the sacred month. Direct us to an act which we should ourselves perform and invite those who live beside us. Upon this the Prophet remarked: I command you to do four things and prohibit you against four acts. (The four deeds which you are commanded to do are): Faith in Allah, and then he explained it for them and said: Testifying the fact. that there is no god but Allah, that Muhammad is the messenger of Allah, performance of prayer, payment of Zakat, that you pay Khums (one-fifth) of the booty fallen to your lot, and I prohibit you to use round gourd, wine jars, wooden pots or skins for wine. Khalaf b. Hisham has made this addition in his narration: Testifying the fact that there is no god but Allah, and then he with his finger pointed out the oneness of the Lord.