(a) Abdullah bin Fadalah (may Allah be pleased with them both) narrates from his father, who said: the Messenger of Allah (peace and blessings be upon him) taught me matters of the religion of Islam. Among them was this: "Guard the five prayers (that is, offer them at their appointed times)." I said: These are the times when I am busy, so give me a comprehensive command which, if I hold to it, will suffice for me. Then he (peace and blessings be upon him) said: "Guard the 'asrayn (the two 'Asr) prayers." This word ('asrayn) was not used in our dialect. I asked: What is meant by 'asrayn? He (peace and blessings be upon him) said: "The two prayers before sunrise and before sunset."
(b) The Shaykh (may Allah have mercy on him) says that his (peace and blessings be upon him) meaning was: guard these prayers by offering them at the beginning of their times. But when the man offered the excuse of his occupations, that he would not be able to pray at the appointed time and there would be delay, he (peace and blessings be upon him) told him specifically about these two prayers that he should offer them at the beginning of their time. And Allah knows best, and success is only from Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2188