Sayyiduna Abu Mas'ud (may Allah be pleased with him) said: Jibril (peace be upon him) came down to the Prophet (peace and blessings be upon him) with the times of prayer and gave him (peace and blessings be upon him) the command, so he (peace and blessings be upon him) prayed Zuhr when the sun had declined; then he prayed Asr when the shadow of everything had become equal to its measure; then he prayed Maghrib when the sun had set; then he prayed al-Atamah, which is Isha, when the redness of twilight had disappeared; then he prayed Subh when the dawn had broken. Then he came to him (peace and blessings be upon him) the next day, and he (peace and blessings be upon him) delayed Zuhr until (the shadow was) the measure of its object, and delayed Asr until (the shadow was) twice its measure; then he prayed Maghrib when the sun had set; then he prayed Isha in the darkness; then he prayed Subh when it had become light. Then he said: "Between these two is the time of prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1709