Musnad Ahmad 1299:1

Yazid ibn Harun narrated to us, he said: Abdullah ibn Umar informed us, from al-Qasim ibn Ghannam, from his family, from his grandmother, Umm Farwah, that she heard the Messenger of Allah, peace and blessings of Allah be upon him, when a man asked him about the best of deeds. The Messenger of Allah, peace and blessings of Allah be upon him, said: "The prayer at the beginning of its time."