Sahl bin Sa'd (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "Whoever guarantees me what is between his two jaws (the tongue) and what is between his two legs (the private parts), I guarantee him Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16671