The Messenger of Allah ﷺ said, "Whoever catches one rakah of the Asr prayer before the sun sets, or one rakah of the Fajr prayer before the sun rises, has caught the prayer." The "sajdah" mentioned here refers to a rakah.
قال رسول الله ﷺ: من أدرك من العصر سجدة قبل أن تغرب الشمس، أو من الصبح قبل أن تطلع فقد أدركها والسجدة إنما هي الركعة.
Muslim narrated it in the Book of Mosques (609) through several routes from Yunus, from Ibn Shihab, that Urwa bin Zubayr told him, from Ayesha, and she mentioned something similar.