Yahya related to me from Malik from Zayd ibn Aslam from Ata ibnYasar and from Busr ibn Said and from al-Araj-all of whom related itfrom Abu Hurayra - that the Messenger of Allah, may Allah bless himand grant him peace, said, "Whoever manages to do a raka of subhbefore the sun has risen has done subh in time, and whoever manages todo a raka of asr before the sun has set has done asr in time."
Arabic/English book reference: Book 1, Hadith 5