Sayyiduna Abu al-Darda' (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "The example of one who acquires knowledge in his youth is like an engraving on stone, and the example of one who learns knowledge in his old age is like one who writes on water."
Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Marwan bin Salim al-Shami, whom Imam Bukhari (may Allah have mercy on him), Imam Muslim and Abu Hatim declared da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العلم / 515