From Sahih Muslim (Hadith 6260):
Iyas reported on the authority of his father:I (had the honour of) leading the white mule on which rode Allah's Apostle (ﷺ) and with him were Hasan and Husain, till it reached the apartment of Allah's Apostle (ﷺ). The one amongst them was seated before him and the other one was seated behind him
Reference: Muslim 6260