Hadith: This world in comparison with the Hereafter is like the amount of water one of you gets when he dips his finger in the sea. Let him see what his finger returns with

عن المُسْتَوْرِد بن شَدَّاد رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : «ما الدنيا في الآخرة إلا مِثْل ما يجعل أحدكم أُصْبُعَهُ في اليَمِّ، فلينظر بِمَ يَرْجع!».
[صحيح] - [رواه مسلم]

Al-Mustawrid ibn Shaddād (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "This world in comparison with the Hereafter is like the amount of water one of you gets when he dips his finger in the sea. Let him see what his finger returns with."
Sahih/Authentic. - [Muslim]

Explanation

Meaning of this Hadīth: If you wish to know the reality of this world in comparison to the Hereafter, then put your finger in the sea and take it out, then look at what it comes back with. What it comes back with is nothing compared to the water in the sea. This is exactly what this world means in terms of its short duration and the perishing of its pleasures in comparison to the Hereafter which is everlasting and its pleasures are eternal. Allah, the Almightyو says: {But little is the enjoyment of the life of this world as compared with the Hereafter} [Sūrat at-Tawbah: 38]. Everything that one is given in this worldly life of riches and pleasures are short-term and mixed with pains and vexations. He brags and boasts about his worldly gains, while they are only temporary. They soon perish and are succeeded with sorrow and regret. Allah says: {And whatever of things you have been given, are only for the enjoyment of worldly life and its adornment. And what is with Allah is better and more lasting; so will you not then use reason?} [Sūrat al-Qasas: 60] What is with Allah of eternal bliss, joyous life, and fancy abodes is better in quality and quantity, and it will be everlasting and forever.