Musannaf Ibn Abi Shayba 13:2548

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, he said: I heard ‘Amr ibn ‘Umar saying: ‘Umar said: "If he asks you for a delay [for the duration of] the milking of a she-camel, then do not grant him a delay," meaning in the exchange of currency (sarf).