It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever catches one rak'ah of Asr before the sun sets, or catches the Subh prayer before the sun rises, his prayer is valid."
In the hadith, "sajdah" (prostration) means a rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1772