Hadith: This is mercy that Allah, the Exalted, has placed in the hearts of His slaves. Verily, Allah shows mercy to those of His slaves who are merciful

عن أسامة بن زيد رضي الله عنهما قال: رُفع إلى رسول الله - صلى الله عليه وسلم - ابن ابنته وهو في الموت، ففاضت عينا رسول الله صلى الله عليه وسلم فقال له سعد: ما هذا يا رسول الله؟! قال: «هذه رحمة جعلها الله تعالى في قلوب عباده، وإنما يرحم الله من عباده الرحماء».
[صحيح] - [متفق عليه]

Usāmah ibn Zayd (may Allah be pleased with him) reported: The son of the daughter of the Messenger of Allah (may Allah’s peace and blessings be upon him) was brought to him while breathing his last. The eyes of the Messenger of Allah (may Allah’s peace and blessings be upon him) overflowed with tears. Thereupon, Sa‘d said to him: "O Messenger of Allah, what is this!" He said: "This is mercy that Allah, the Exalted, has placed in the hearts of His slaves. Verily, Allah shows mercy to those of His slaves who are merciful."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Usāmah ibn Zayd, who was nicknamed "the beloved of the Messenger of Allah", reported that one of the daughters of the Messenger of Allah (may Allah’s peace and blessings be upon him) sent a messenger to him to inform him that her son was dying and that she requested that he comes to her house. Her messenger conveyed this to the Messenger of Allah (may Allah’s peace and blessings be upon him) who said to him: "Order her to be patient and seek Allah’s reward, for, indeed, what Allah takes belongs to Him, and what He gives belongs to Him, and everything with Him has a determined fixed term." The Prophet (may Allah's peace and blessings be upon him) ordered the man whom his daughter had sent to order his daughter, the mother of that child, with these words: "What Allah takes belongs to him, and what He gives belongs to Him." This is a magnificent statement. If all things belong to Allah, then if He takes something from you, it is His own possession, and if He gives something to you, it is His own possession. So, how could you be discontent when He takes something from you that belongs to Him? For this reason, it is part of the Sunnah for a person to say when afflicted with a calamity: "Truly, to Allah we belong and, truly, to Him we shall return," which means: we are Allah’s possessions and He does with us as He wishes. Likewise, if He takes something we love from our hands, it belongs to Him. What He takes belongs to Him and what He gives belongs to Him. Even the thing which He gives to you does not belong to you; it belongs to Allah. That is why you may not act freely concerning what Allah has given to you except in the manner which He allowed you. And this is proof that our possession of what Allah gives us is limited and temporary. As to his statement: "and everything with Allah has its determined fixed term," if you are certain of this; that what Allah takes belongs to Him and what He gives belongs to Him, and that everything has its appointed time with Him, then you will be content with what you have. The last part of his statement means that it is not possible for a person to change what has already been written and bound to a certain time limit, neither by advancing nor delaying, as Allah says (what means): {For every nation is a term. When their time has come, then they will not remain behind an hour, nor will they precede (it)"} [Sūrat Yunus: 49]. So, if something is predestined and can neither be advanced nor delayed, there is no benefit in anguish and discontent, because even if you feel anguish or discontent, you will not change anything of what is predestined. Then, the messenger conveyed to the daughter of the Prophet (may Allah's peace and blessings be upon him) what he ordered him to tell her. However, she again requested his presence. So the Prophet (may Allah's peace and blessings be upon him) along with a group of his Companions, went to her home. The boy was brought to him with his breath rattling in his chest. The Messenger of Allah (may Allah's peace and blessings be upon him) wept and tears filled his eyes. Sa‘d ibn ‘Ubādah, the chief of the Khazraj Tribe, who was with him, said to him: "What is this?!" He thought that the Messenger of Allah (may Allah’s peace and blessings be upon him) wept out of anguish and grief. So, the Prophet (may Allah's peace and blessings be upon him) said: "This is mercy," meaning: I wept out of mercy for the child, not out of impatience with fate. Then he said: "Verily, Allah shows mercy only to His slaves who are merciful." This statement is proof of the permissibility of weeping out of mercy towards the afflicted person.