(a) It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Remain steadfast, though you will not be able to count up the good deeds; and know that the best of your deeds is the prayer, and the believer always remains in a state of ablution (wudu)."
(b) Imam al-Shafi'i (may Allah have mercy on him) says: The prayer offered at the beginning of its time is the best deed of the son of Adam. And he mentioned the earlier hadith in which the Prophet (peace and blessings be upon him) was asked about the best deed, and he (peace and blessings be upon him) said: "Offering the prayer at the beginning of its time."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2150