It is narrated from Sayyidah Umm Farwah (may Allah be pleased with her); she said: The Messenger of Allah (peace and blessings be upon him) was asked, while I was listening: Which deed is best? He (peace and blessings be upon him) said: "Offering the prayer at the beginning of its time."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21672