It is narrated from Ata bin Abi Rabah (may Allah have mercy on him) that the minimum duration of menstruation is one day. Abu Ibrahim (may Allah have mercy on him) said: The Imam of piety, Ahmad bin Hanbal (may Allah have mercy on him), held to these two narrations.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15408