Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "He who weeps out of fear of Allah, will not enter the Hell till the milk returns into the udder; and the dust raised while fighting in the path of Allah and the smoke of Hell will not come together."
The Messenger of Allah (may Allah's peace and blessings be upon him) stated that the person who weeps out of fear of Allah will not enter the Hellfire, because fear of Allah usually arises from obedience to Him and refraining from sins. He used the metaphor "until milk returns into the udders" to indicate impossibility. Similarly, the dust raised when someone is fighting for the sake of Allah will prevent him from being exposed to the smoke of the Hellfire, as they (that dust and that smoke) are two opposites, just as the worldly life and the Hereafter are two opposites.