It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Whoever catches one rak'ah of the Fajr prayer before sunrise has caught that prayer, and whoever catches one rak'ah of the Asr prayer before sunset has caught that prayer.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9183