Sahih Muslim Book 4, Hadith Number 1820.

Chapter : Do not get into the habit of observing prayer at the time of the rising sun and at the time of its setting.

Mukhtar b. Fulful said: I asked Anas b. Malik about the voluntary prayers after the afternoon prayer, and he replied: ‘Umar struck hit hands on prayer observed after the ‘Asr prayer and we used to observe two rak’ahs after the sun set before the evening prayer during the time of the Apostle of Allah (may peace be upon him). I said to him: Did the Messenger of Allah (may peace be upon him) observe them? He said: He saw us observing them, but he neither commanded us nor forbade us to do so.