Abu Mijlaz said that a funeral passed by Ibn Abbas and al-Hasan bin Ali (may Allah be pleased with them both). One of them stood up and the other did not stand, so one of them said: "Did the Prophet (peace and blessings be upon him) not stand?" The other said: "Yes indeed, but later he sat."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6889