(30) باب مَنْ أَدْرَكَ رَكْعَةً مِنَ الصَّلاَةِ فَقَدْ أَدْرَكَ تِلْكَ الصَّلاَةَ
Abu Huraira reported the Messenger of Allah (ﷺ) as saying:
He who finds one rak'ah of the prayer with the Imam, he in fact finds the prayer.
Reference: Sahih Muslim 607 b
In-book reference: Book 5, Hadith 206
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1261