Sayyiduna Bilal bin al-Harith (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "A man speaks a word of good whose outcome he does not know, and Allah, because of it, writes for him His pleasure until the day he meets Him. And a man speaks a word of evil whose outcome he does not know, and Allah, because of it, writes against him His displeasure until the day he meets Him."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1130