From Sahih Muslim (Hadith 1377):
Abu Huraira reported:The Messenger of Allah (ﷺ) said: He who finds (gets) a rak'ah of the afternoon (prayer) before the setting of the sun, he in fact gets (the full prayer), and he who gets a rak'ah of the morning (prayer) before the rising of the sun he in fact gets (the full prayer)
Reference: Muslim 1377