Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5953
Iyas reported on the authority of his father: I (had the honour of) leading the white mule on which rode Allah's Apostle (may peace be upon him) and with him were Hasan and Husain, till it reached the apartment of Allah's Apostle (may peace be upon him). The one amongst them was seated before him and the other one was seated behind him.