Musannaf Ibn Abi Shayba 3:842

Waki’ narrated to us, he said: Al-‘Umari narrated to us, from Qasim ibn Ghannam, from some of his mothers, from Umm Farwah, that she asked the Prophet, may Allah bless him and grant him peace: "Which deed, or which prayer, is best?" He said: "The prayer at the beginning of its time."