It's likely you're thinking of one of his famous poems:
* "My Last Farewell" (Mi Ultimo Adios): This poem, written in Spanish, is considered Rizal's most famous work. He wrote it in his prison cell in Fort Santiago, Manila, just hours before his execution on December 30, 1896.
* "A La Juventud Filipina" (To the Filipino Youth): This poem, written in Spanish, was dedicated to the Filipino youth. Rizal wrote it in 1879 while he was studying at the Ateneo Municipal de Manila.
It's important to be precise when discussing Rizal's works, as he wrote many poems, and the specific title can help pinpoint the exact poem you're interested in.