Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him): "Whoever catches one rak'ah of Subh before the sun rises and one rak'ah after it rises has caught Subh (the prayer); and whoever catches one rak'ah of Asr before the sun sets and three rak'ahs after it sets has caught Asr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1775