Please help me find this piece!

I've had it on my mind and just can't figure out what it is. It's likely Italian or Latin, and starts with something like O si.
It's probably not the right key, but it goes like:
F#4 Db5 Ab4 A4 A4

F#4 Db5 Ab4 A4 B4 A4 F#4 F#4