Musannaf Ibn Abi Shaybah

Hazrat Abu Huraira narrates from the Noble Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: Whoever catches one rak'ah of Asr prayer before sunset, indeed he has caught the whole prayer. And whoever catches one rak'ah of Fajr prayer before sunrise, indeed he has caught the whole prayer. And (Imam) Abu Hanifa's opinion has been mentioned that: When a person has performed one rak'ah of Fajr and the sun rises, then this Fajr will not suffice for that person.