Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "If a man fasts a voluntary fast for the sake of Allah, and then he is given the earth's fill of gold, even then he will not receive his full reward before the Day of Reckoning."
Hadith Referenceالمعجم الأوسط / باب العين / 4869
Hadith Takhrij«أخرجه أبو يعلى فى «مسنده» برقم: 6130، وابن حجر فى «المطالب العالية» برقم: 1002»