Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

and (one of the narrators) Shubah said: "Sometimes he (Qatadah, his teacher) narrated it as a Marfu report and sometimes he did not" - "The time for Zuhr prayer is until Asr comes, and the time for Asr prayer is until the sun turns yellow. The time for Maghrib is until the twilight disappears, and the time for 'Isha' is until the night is halfway through, and the time for Subh is until the sun rises." (Sahih)