Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) said: I set out with the Messenger of Allah (peace and blessings be upon him) on the expedition of Tabuk. I saw a moment of privacy and seized it, so I urged my camel towards him (peace and blessings be upon him) until I was riding alongside him (peace and blessings be upon him), and I said: O Messenger of Allah, teach me a deed that will admit me into Paradise. He (peace and blessings be upon him) said: "You have asked about something tremendous, yet it is easy for the one for whom Allah makes it easy: that you worship Allah and do not associate anything with Him, establish the obligatory prayer, pay the obligatory zakah, and fast Ramadan." Then he (peace and blessings be upon him) rode on and I rode on.
He said: "And if you wish, I will tell you about the gates of good: fasting is a shield, charity wipes out sins, and a man's standing (in prayer) in the depths of the night." Then he (peace and blessings be upon him) recited this verse: "Their sides forsake their beds" [Al-Sajdah: 16]. Then he (peace and blessings be upon him) rode on and I rode on.
And he said: "Shall I not tell you the head of this matter, its pillar and its highest peak? Jihad in the path of Allah." Then he (peace and blessings be upon him) rode on and I rode on.
He said: "If you wish, I will tell you of that which has more control over people than all of that." (The narrator said:) He (peace and blessings be upon him) fell silent for a little while, and I looked around and saw a rider. I thought he might come to him (peace and blessings be upon him) and occupy him, so I said: O Messenger of Allah, what is it that has more control than all of that? He (peace and blessings be upon him) pointed with his hand to his tongue. I said: O Messenger of Allah, will we be held accountable for what we say? He (peace and blessings be upon him) said: "May your mother be bereaved of you, O Mu'adh bin Jabal! Whatever you say is either for you or against you. Does anything topple people on their noses into the fire of Hell except the harvests of their tongues?"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17427