Alqamah narrates that a man asked Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) about the time of Zuhr. He said: "When a man's shadow draws in beneath his feet, and this time lasts until the shadow of everything becomes equal to it." He asked him about the time of Asr, and he said: "Pray it while the sun is still white and bright." And he asked about the time of Maghrib, and he said: "When the sun has set."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9161