Musnad Ahmad 31:118

Mu'awiyah ibn 'Amr narrated to us, Abu Ishaq al-Fazari narrated to us, from al-Awza'i, from al-Zuhri, from 'Ata' ibn Yazid, from Abu Sa'id al-Khudri, who said: A man came to the Prophet, may Allah bless him and grant him peace, and asked him about the Hijrah (migration). He said: "Woe to you, the matter of the Hijrah is severe. Do you have camels?" He said: "Yes." He said: "Do you pay their Zakat (obligatory charity)?" He said: "Yes." He said: "Do you give some of them as a gift (to be milked)?" He said: "Yes." He said: "Do you milk them on the day they come to the water?" He said: "Yes." He said: "Then work from beyond the seas, for Allah will not let any of your deeds go to waste."