Musnad Ahmad 31:711

Waki’ narrated to us, ‘Abd al-Hamid ibn Bahram narrated to us, from Shahr ibn Hawshab, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no prayer after the dawn [Fajr] until the sun rises, and no prayer after the afternoon [‘Asr] until it sets."