Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1814
'A'isha said that the Messenger of Allah (may peace be upon him) did not abandon observing two rak'ahs after 'Asr, but she reported Allah's Messenger (may peace be upon him) as saying: Do not get used to observe prayer at the time of the rising sun and at the time of its setting and (exhorted the Muslims) to pray at their times.