Sayyiduna Fadalah al-Laythi (may Allah be pleased with him) narrates: The Messenger of Allah (peace and blessings be upon him) taught me, and among the things he taught me was that he said: "Guard the five prayers." I said: These are hours in which I have occupations, so tell me something comprehensive which, if I do it, will suffice for me. He (peace and blessings be upon him) said: "Guard the two Asrs." I said: What are the two Asrs? He (peace and blessings be upon him) said: "A prayer before the sun rises, and a prayer before it sets."
Hadith Referenceالمعجم الكبير للطبراني / باب الفاء / 15917