(a) Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him): "The time of Zuhr is as long as the time of Asr has not come; the time of Asr is as long as the sun has not turned yellow; the time of Maghrib is as long as the redness of twilight has not disappeared; the time of Isha is until the middle of the night; and the time of Fajr is as long as the sun has not risen." (b) In a narration from Qatadah: "The time of Asr is as long as the sun has not turned yellow and its first edge has not sunk." Its chain has already been mentioned.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1715