Sayyiduna Ibn Mas'ud (may Allah be pleased with him) reports that once I asked the Prophet (peace and blessings be upon him): "Which deed is most beloved to Allah?" The Prophet (peace and blessings be upon him) replied: "Prayer at its proper time." I asked: "Then what?" He said: "Kindness to parents." I asked: "Then what?" He said: "Striving in the way of Allah." If I had asked further, the Prophet (peace and blessings be upon him) would have answered my questions as well.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4243