Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him): "When one of you catches one rak'ah of Asr before the sun sets, let him complete his prayer; and when he catches one rak'ah of the Subh prayer before the sun rises, let him complete his prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1774