Weeping is a Mercy which Allah puts in the Hearts of His Servants

On the authority of Usamah ibn Zayd (radhyAllahu ‘anhu) who said:

"We were with the Prophet (sallAllahu ‘alaihi wa sallam) when one of his daughters send a messenger calling for him, informing him that her small child or her son was dying. So the Prophet (sallAllahu ‘alaihi wa sallam) told the messenger: "Return to her and inform her that whatever Allah takes and gives belongs to Him and that everything has an appointed term, so let her be patient and anticipate Allah's reward in the hereafter". Later the messenger returned to the Prophet (sallAllahu ‘alaihi wa sallam) and said, "Verily she has sworn by Allah that you come to her". So the Prophet (sallAllahu ‘alaihi wa sallam) got up and so too did Sa'ad ibn 'Ubadah and Mu'adh ibn Jabal, and I too went with them. The small boy was lifted up to the Prophet (sallAllahu ‘alaihi wa sallam) and let out a groaning noise[1], as if he was breathing his last breath. Thereby the Prophet's (sallAllahu ‘alaihi wa sallam) eyes overflowed with tears. Upon this Sa'd asked: "What is this, O Messenger of Allah?" He (sallAllahu ‘alaihi wa sallam) replied: "This is a mercy which Allah puts in the hearts of His servants. And verily Allah shows mercy to those of His servants who are merciful" (Reported y al Bukhari, Muslim)

The Weeping of the Prophet Muhammad (sallAllahu ‘alaihi wa sallam):

Ibn al Qayyim said in Zad al Ma'ad:         

"As for the weeping of the Prophet (sallAllahu ‘alaihi wa sallam) it was in the same degree as his laughter. He wouldn't sob loudly and raise his voice, just like his laughter wasn't loud. However his eyes would fill up with tears, until they flowed out, and you would hear the sound like that of a whistling kettle coming from his chest. He would weep out of mercy for the dead, out of fear and compassion for his ummah, out of deep fear of Allah, upon listening to the Qur'an. And it was a weeping of longing, love and exaltation, accompanied by fear and khashyah".

On the authority of 'Abdullah ibn Mas'ood (radhyAllahu ‘anhu) who said:        

"The Messenger of Allah (sallAllahu ‘alaihi wa sallam) told me: "Recite to me!" So I recited Surah an-Nisa' until I reached: "How will it be then, when We bring from each nation a witness, and bring you (O Prophet Muhammad) as a witness against these people?" (Surah an-Nisa' 4:41) Then I looked towards him and behold! I saw that his eyes were overflowing with tears" (Reported y al Bukhari, Muslim and others)[2]

On the authority of 'Ali (radhyAllahu ‘anhu) who said,    

"We did not have a horseman with us on the day of Badr except al-Miqdad. Everyone amongst us was sleeping except for the Messenger of Allah (sallAllahu ‘alaihi wa sallam) who was under a tree, praying and weeping until the morning." (Reported y Ibn Khuzayman in his Sahih)

On the authority of 'Abdullah ibn 'Amr (radhyAllahu ‘anhu) who said:

"One day during the lifetime of the Messenger of Allah (sallAllahu ‘alaihi wa sallam) the sun eclipsed, so he stood and prayed, until it seemed as though he wouldn't go into rukoo, then he went into rukoo (for such a long time) that it seemed as if he wouldn’t raise his head, then he raised his head. He remained (in that position) and it seemed as if he wouldn't go into sujood, then he made sajdah, and (he remained in that position) until it seemed as if he wouldn't raise his head, then he raised his head and it seemed as if he wouldn't go into sujood again, then he made sajdah, until it seemed as if he wouldn't raise his head again. Then he began to breathe heavily and weep, saying, "Lord, didn't You promise me that You wouldn't punish them while I am amongst them? Lord didn’t You promise me that You wouldn't punish them whilst they seek forgiveness from you, and we seek forgiveness from you?". When he had finished praying two rak'ah, the sun eclipse had cleared and he (sallAllahu ‘alaihi wa sallam) got up and praised and glorified Allah and then he said: "The sun and the moon are two signs from among the signs of Allah, they do not eclipse because of the death or life of anyone. So if you see them eclipse, then hasten to the remembrance of Allah" (Reported by an-Nasa'i, Abu Dawud)

On the authority of al-Bara ibn 'Aazib (radhyAllahu ‘anhu) who said:    

"While we were with the Messenger of Allah (sallAllahu ‘alaihi wa sallam) he suddenly looked towards a group (of people) and said: "For what reason have they gathered here?" It was said, "In order to dig a grave". So the Messenger of Allah (sallAllahu ‘alaihi wa sallam) became alarmed and startled and he quickly went ahead of the Companions until he reached the grave, then he knelt upon it and I turned my face towards him (sallAllahu ‘alaihi wa sallam) in order to see what he was doing. He (sallAllahu ‘alaihi wa sallam) cried until the earth became wet with his tears, then he turned to us and said, "O my brothers! Prepare for a day like this"" (Reported by al Bukhari in at-Tarikh, Ibn Majah, Ahmad and others, hasan)

On the authority of 'Abdullah ibn ash-Shikh-kheer (radhyAllahu ‘anhu) who said:      

"I saw the Messenger of Allah (sallAllahu ‘alaihi wa sallam) praying with us, and I heard the sound of his weeping coming out of his chest, which was like the sound of a boiling pot". (Reported y Abu Dawud, an Nasa’i, at-Tirmidhi in ash-Shama'il, Ibn Hajar in al-Fath, strong chain of narration, declared authentic by Ibn Khuzaymah, Ibn Hibbah and al-Hakim).

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[1] Al qa'qa': the movement of something, from which a noise is heard. The meaning intended here is agitation and movement. He (ie. the narrator) meant each time the child entered upon a particular state it did not take long before he entered upon another which brings him closer to death. Refer to an-Nihayah.

[2] An explanation of this noble verse has been mentioned in a hadith related by Abu Sa'id (radhyAllahu ‘anhu) who said that the Messenger of Allah (sallAllahu ‘alaihi wa sallam) said: "A prophet will come and with him will be two men, and (another) prophet will come and with him will be three men, and more and less than that. It will be said to him: "Did you convey (the message) to your nation?" he will reply: "Yes" then his nation will be called, and it will be said to them: "Was the message conveyed to you?" and they will reply: "No". So it will be said (to that prophet) "Who will testify for you?" He will reply, "Muhammad (sallAllahu ‘alaihi wa sallam) and his nation". So the nation of Muhammad (sallAllahu ‘alaihi wa sallam) will be called and it will be said to them: "Were you informed of this?" and they will reply: "Yes". So it will be asked: "How did you come to know about that?" and they will reply: "Our Prophet (sallAllahu ‘alaihi wa sallam) informed us that the messengers (before him) had conveyed the message, and we believe him". He (Abu Sa'eed) said: "So that is what Allah (the Most High) means when He says in the Qur’an: "...and thus We have made you (Muslims, real believers) a just nation that you be witnesses over mankind, and the Messenger (Prophet Muhammad) be a witness over you..." (Surah Al-Baqarah, 2:143.) (Ibn Majah, Ahmad, and al Bukhari)