Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him): Which deed is most beloved to Allah Almighty? He (peace and blessings be upon him) said: "That you die while your tongue is moist with the remembrance of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17317