It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever catches one rak'ah of Asr before the sun sets has caught the Asr prayer, and whoever catches one rak'ah before the sun rises has indeed caught the Fajr prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1717