Abu Hurayrah (may Allah be pleased with him) reported that the Messenger (may Allah's peace and blessings be upon him) said: "Whoever catches one Rak‘ah before sunrise, he has caught the Fajr prayer, and whoever catches one Rak‘ah of the ‘Asr prayer before sunset, he has caught the ‘Asr prayer." Authentic hadith - Narrated by Bukhari & Muslim