(30) باب مَنْ أَدْرَكَ رَكْعَةً مِنَ الصَّلاَةِ فَقَدْ أَدْرَكَ تِلْكَ الصَّلاَةَ
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: Sahih Muslim 608 c
In-book reference: Book 5, Hadith 211
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1266