How to boot Samsung Galaxy SL( GT-I9003) in download mode
Once you have power off the phone, press and Hold Volume DOWN and HOME button. Make sure you have hold volume up, and don't press in the middle of the volume buttons.
Now keep pressing these two buttons(VOL DOWN + HOME) and press POWER button. Keep pressing these Buttons, until you see the screen shown below.Once you see download mode screen release all the buttons. Now you have entered to download mode.