It is narrated from one of the female Companions among the women who gave the pledge of allegiance that the Prophet (peace and blessings be upon him) was asked: Which deed is best? He (peace and blessings be upon him) said: "Faith in Allah, then offering the prayer at the beginning of its time."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21673