Musnad Ahmad 288:25

Ali ibn Hafs and Husayn ibn Muhammad narrated to us, they said: Shu'bah narrated to us, from al-Nu'man ibn Salim, he said: I heard Ibn 'Amr ibn Aws narrating from his grandfather, Aws ibn Abi Aws, that he "saw the Prophet, may Allah bless him and grant him peace, performing ablution, and he *istawkafa* three times." He said: I said: What does *istawkafa* three times mean? He said: He washed his hands three times.