It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (S) said: “A man who weeps for fear of Allaah will not enter Hell until the milk goes back into the udder, and dust produced (when fighting) for the sake of Allaah and the smoke of Hell will never coexist.”
[Narrated by al-Tirmidhi, 1633; al-Nasaa’i, 3108; classed as saheeh by al-Albaani]
“until the milk goes back into the udder” is a metaphor for it being impossible, as in the verse where Allaah says (interpretation of the meaning): “and they will not enter Paradise until the camel goes through the eye of the needle”
[The Noble Quran, Al-A’raaf 7:40]. Tuhfat al-Ahwadhi.