Musnad Abu Ya'la al-Mawsili 155:127

Abu Sa'id al-Ashajj narrated to us, Abdullah ibn Idris al-Awdi narrated to us, from Ubaydullah ibn Umar, from al-Zuhri, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever catches a rak'ah of the prayer, he has caught it entirely."