What is the ruling on saying "May your death make you forget," and the ruling on listening to it?

Praying for a long life is permissible. It is better, however, to qualify it with what benefits the person being prayed for, such as being in obedience to Allah, based on the hadith narrated by Anas (may Allah be pleased with him). Ambiguous phrases in supplication should be avoided, and the fulfillment of needs should be sought from Allah, attributing the action to Him, the Exalted.