Musnad Ahmad 27:370

Rawh narrated to us, he said: Al-Awza'i narrated to us, from Al-Muttalib ibn 'Abdullah ibn Hantab, who said: Ibn 'Umar used to perform ablution three times, attributing it to the Prophet, may Allah bless him and grant him peace. And Ibn 'Abbas used to perform ablution once, attributing it to the Prophet, may Allah bless him and grant him peace.