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 the morning prayer before the sun rises has caught the morning prayer, and whoever catches one rak'ah of Asr before the sun sets has caught Asr."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1812