Musnad Ahmad 32:1002

'Abd al-A'la ibn 'Abd al-A'la narrated to us, Yahya ibn Abi Ishaq narrated to us, from Anas ibn Malik, who said: "We went out with the Messenger of Allah, may Allah bless him and grant him peace, from Medina, and he began to pray two rak'ahs [at a time] until he returned." Yahya said: So I said to Anas: "How long did he stay?" He said: "Ten [days]."