Sayyiduna Abdullah bin Mu'awiyah al-Ghadiri (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever does three things has tasted the sweetness of faith: one who worships Allah alone, knowing that there is no god but Allah; second, who pays the zakah of his wealth every year willingly, and does not give in zakah an old, inferior, blameworthy or sick (animal), but pays it from his middling wealth. Allah Almighty does not ask you for the best of it, nor does He command you to give the worst of it; and (third) a servant purifies his own soul." A man said: O Messenger of Allah! What is the purification of the soul? He (peace and blessings be upon him) said: "That he knows with certainty that Allah is with him wherever he is."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7275