Should I tell him I saw what he has on his phone? - EDIT

Today I picked up my boyfriend of 8 years phone to call my phone because I couldn’t find it. (We both use each other’s phone not a big deal) . When I swiped the phone to unlock it there was a text already opened to his sister. On the text it send “I finally found the right ring I think she will love it” and a picture of a engagement ring....

now before you all go all huffy and puffy on me I was not snooping the text was already open. when I saw it I immediately threw the phone down and tried to unsee but I couldn’t unsee what I already saw.

So now should I tell him I totally ruined the surprise or keep it a secret so he can still surprise me because I know him and he would enjoy surprising me.

Edit- I can’t help but feel somewhat guilty keeping this secret. Part of me says don’t tell him and ruin this, and the other part says it’s wrong to keep it secret

Would you consider this a ok lie?