Mujahid narrates from Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) stayed forty days in Khaybar, praying two rak'ahs each.
Al-Hasan bin Umarah alone narrated it, and he is not relied upon as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5475