sarve bhavantu sukhina ḥ

[Dominik Wujastyk]

... sarve santu nirāmayaḥ

I've been asked where this famous sentence comes from, historically, and I 
don't know.

The Mahāmantrānusāriṇī (thanks to Peter Skilling and GRETIL) has it:

C.3.16. sarve satvāḥ sarve prāṇāḥ sarve bhūtāś ca kevalāḥ /
sarve vai sukhinaḥ santu sarve santu nirāmayāḥ /
sarve bhadrāṇi paśyantu mā kaścit pāpam āgamat //

But people usually ascribe it to one of the upaniṣads, where as far as I 
can see it doesn't appear.  Is it known from other, older sources?

Thanks,
Dominik