Usamah ibn Zayd narrated:
The daughter (Zainab) of the Prophet (pbuh) sent a messenger to the Prophet (pbuh) asking him to come as her child was about to dying. However, the Prophet (pbuh) returned the messenger and asked him to carry his greetings to her and say:
'Whatever Allah takes is for Him and whatever He gives is for Him. Everything with Him has a limited time and so she should be patient and hope for Allah's reward.'
She again sent a message to him, insisting that he should come. The Prophet (pbuh) stood up and went to her, and so did his companions; Sa'id ibn Ubadah, Mu'ath ibn Jabal, Ubay ibn Ka'ab, Zayd ibn Thabit and some other men. When the child was brought to God's Messenger (pbuh); its body trembling and moving; the eyes of God's Messenger began shedding tears. Upon this, Sa'd asked:
'O Messenger of God! What is the weeping and shedding of tears?'
Prophet Muhammad (pbuh) replied:
'It is mercy which Allah has put into the hearts of His servants. Allah is merciful only to those of His servants who are merciful to others'