Is it obligatory to say 'sami`a allahu liman hamidah'?

When following an imam, sometimes I forget to mention "Allah hears whoever praises Him" and only say "Our Lord, all praise is Yours". What is the validity of my prayer according to Shafi'i?

In the Name of Allah, Most Merciful and Compassionate

In the Shafi`i school, it is recommended for the follower to say sami`a allahu liman hamidah as well as rabbana lakal hamd when following the imam. Neither of these formulas, however, are integral parts of the prayer, and omitting them would not invalidate one's prayer.

In other words, if you forget to utter either of these formulas, you have missed an opportunity of eternal reward in the next life, but you haven't committed a sin, and neither do you have to repeat your prayer.

And Allah Most High knows best.

Hamza.