´It was narrated from 'Aishah that:` The Messeenger of Allah said: "Whoever catches one Rak'ah of the Subh before the sun rises, then he has caught it, and whoever catches one Rak'ah of the 'Asr before the sun sets, then he has caught it." (Sahih)Another chain with similar wording.
Hadith Referenceسنن ابن ماجه / كتاب الصلاة / 700
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم