This (line) represents man, and this (line) represents the end of his life, and while he is as such, the closest line overtakes him
Sahih/Authentic with its two versionsعن أنس رضي الله عنه قال: خطَّ النبيُّ صلى الله عليه وسلم خُطُوطاً، فقال: "هذا الإنسان، وهذا أجَلُهُ، فبينَما هو كذلكَ إذ جاءَ الخَطُّ الأقْرَبُ". عن ابن مسعود رضي الله عنه قال: خطَّ النبيُّ صلى الله عليه وسلم خطاً مربعاً، وخطَّ خطاً في الوسطِ خارجاً منه، وخطَّ خططاً صغاراً إلى هذا الذي في الوسطِ من جانبِه الذي في الوسطِ، فقال: «هذا الإنسانُ، وهذا أجلُهُ محيطاً بِه -أو قد أحاطَ بِه- وهذا الذي هو خارجٌ أملُهُ، وهذه الخُطَطُ الصِّغَارُ الأعْرَاضُ، فإن أخطَأهُ هذا، نَهَشَهُ هذا، وإن أخطَأهُ هذا، نَهَشَهُ هذا».
Anas (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) drew lines (in the sand) and said: "This (line) represents man, and this (line) represents the end of his life term, and while he is as such, the nearest line overtakes him." Ibn Mas‘ūd (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) drew a square and drew a line in the middle that extended out of it. He drew some small lines on both sides of that central line, and said: "This is man (the line in the middle), and this is his life span encompassing him (the square). This line extending out of the square is his hope, and these small lines are the troubles and accidents that befall him. If this one misses him, that one snaps at him, and if this one misses him, that one snaps at him."
Explanation
In both Hadīths, there is an example that illustrates man's hope, his lifespan, all the things to which he is exposed, and his death as a result of one of them. But, if he escapes all of them, then his death will come to him when his lifespan ends. These lines represent the various afflictions that confront him, so if he avoids one, he cannot avoid another, and if he escapes all of them safely, without being afflicted with a disaster such as illness or loss of property or otherwise, then surely he will be overtaken by death. That is, if one does not die due to a cause, then he will surely die as a result of the expiration of his lifespan. So, while a person is in such a state (among these afflictions), the closest line will come to him, which is the end of his life. The Hadīths imply encouragement of having less hope in the worldly life and preparing oneself for the suddenness of death. The verb 'snap (at)', which in Arabic describes the sting of venomous creatures, is used as a metaphor to emphasize fatal affliction.
Source · Narrated by Bukhari
From the Hadith Encyclopedia (hadeethenc.com).