It is narrated from Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "None of you should say, 'My soul has become foul'; rather, let him say, 'My soul has become heavy.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1525