Narrated Abu Said Al-Khudri:

Allah's Messenger (peace be upon him) 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 Messenger (ﷺ)?" He (the Prophet (peace be upon him) ) replied, "It is the Religion."

Reference : Sahih-Bukhari 23
In Book Reference : Book 2, Hadith 16