SAHIH BUKHARI

Volume 1, Book 2, Number 22:

Narrated Abu Said Al-Khudri:

Allah's Apostle said, "While I was sleeping I saw (in a dream) some people wearing shirts of which some were reaching up to the breasts only while others were even shorter than that. Umar bin Al-Khattab was shown wearing a shirt that he was dragging." The people asked, "How did you interpret it? (What is its interpretation) O Allah's Apostle?" He (the Prophet ) replied, "It is the Religion."