Sahih Bukhari
Sahih Bukhari Volume 001, Book 010, Hadith Number 555.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 10. Times of the Prayers
Narated By 'Umar : "The Prophet forbade praying after the Fajr prayer till the sun rises and after the 'Asr prayer till the sun sets."