Sunan Ibn Majah — The Book of Purification and its Sunnah — Hadith No: 512
Translation
It was narrated that Abu Ghutaif Al-Hudhali said:"I was listening to 'Abdullah bin 'Umar bin Khattab in the mosque, and when the time for prayer came, he got up, performed ablution, and offered prayer, then he went back to where he had been sitting. When the time for 'Asr (Afternnon prayer) came, he got up, performed ablution, and offered prayer, then he went back to where he had been sitting. When the time for Maghrib (Sunset prayer) came, he got up, performed ablution, and offered prayer, then he went back to where he had been sitting. I said: 'May Allah improve you (i.e., your condition) Is it obligatory or Sunnah to perform ablution for every prayer?' He said: 'Did you notice that?' I said: 'Yes.' He said: 'No (it is not obligatory). If I perform ablution for Morning prayer I can perform all of the prayers with this ablution, so as long as I do not get impure. But I heard the Messenger of Allah say: "Whoever performs ablution while he is pure, he will have ten merits." So I wanted to earn the merits