The Shanti Mantra for Shri Rudram is as follows:
हरि: ॐ इडादेवहुर्मनुर्यग्न्यनिब्रुहस्पतिरुक्तथामदानि स(गुन०)सिशदविश्वेदेवा: सुक्तवाच: प्रुथिविमातर्मा मा हि(गुन०)सि: मघुमनिश्ये मघुजनिश्ये मधुवक्श्यमि मधुवदिश्यमि मधुमतिं देवेभ्यो वाचुमुद्यस्(गुन०)सुश्रूशेणयामनुशयेभ्यस्तम् मा देवा अवन्तु शोभायै पितरोनुमदन्तु॥
but I cannot find its source nor its true meaning. Please give me meaning and location in scriptures.
It is (Shanti patha of) Shri Rudram Chamakam From Yajurveda
Yajurveda has two primary version:
There are four shakha of Krishna Yajurveda
And Taittiriya Shakha constitue Shri Rudram Chamakam. (Accroding to wikipedia):
The Taittiriya Samhita— (TS) which consists of 8 books or kaandas, subdivided in chapters or prapathakas, further subdivided into individual hymns. Some individual hymns in this Samhita have gained particular importance in Hinduism; e.g. TS 4.5 and TS 4.7 constitute the Shri Rudram Chamakam, while 1.8.6.i is the Shaivaite Tryambakam mantra.
You can see Shri Rudram Chamakam Lyrics at HinduLiterature.org in various languages.
Also from SanskritDocuments.org:
From: gloryofhinduism.blogspot.com
Chamakam Shanti patha
Idaa devahuurmanuryagyaniirbrihaspatirukthaamadaani shasishhadvishvedevaah suuktavaachah prithiviimaatarmaa maa hisiirmadhu manishhye madhu janishhye madhu vakshyaami madhu vadishhyaami madhumatiim devebhyo vaachamudyaasa shushruushhenyaam manushhyebhyastam maa devaa avantu shobhaayai pitaroanumadantu
The Divine Gods are imploved through the mantras of Kamadhenu.
Manu does the Sacrifices, Bruhaspati recites the pleasant mantras.
Let these praises of Visva devas and mother earth save me from sufferance.
Let my pleasant thoughts bring forth pleasant actions and the resultant enjoyable fruits.
Let my Joyous offerings bear fine and worthy speech and my words please the divinity, make men lend their ears to gladden and Gods enlighten me and invoke my speech very powerful and my fore fathers feel excellently glad over it and bless me to perpetuate it.
There is pdf also available at Here.
The mantra you mentioned isn't actually part of the Rudram. The Rudram and Chamakam constitute Prapathaka 5 and Prapathaka 7 of the Fourth Kanda of the Taittiriya Samhita of the Yajur Veda. In contrast, the mantra you're looking for is from Prapathaka 3 of the Third Kanda of the Taittiriya Samhita:
iii.3.2
vāyúr hiṃkartā́gníḥ prastotā́ prajā́patiḥ sā́ma bŕ̥haspátir udgātā́ víśve devā́ upagātā́ro marútaḥ pratihartā́ra índro nidʰánaṃ té devā́ḥ prāṇabʰŕ̥taḥ prāṇám máyi dadʰatu |
etád vái sárvam adʰvaryúr upākurvánn udgātŕ̥bʰya upā́karoti té devā́ḥ prāṇabʰŕ̥taḥ prāṇám máyi dadʰatv íty āhaitád evá sárvam ātmán dʰatte |
íḍā devahū́r mánur yajñanī́s |
bŕ̥haspátir uktʰāmadā́ni śaṁsiṣat |
víśve devā́ḥ ||
súktavā́cas |
pŕ̥tʰivi mātar mā́ mā hiṁsīs |
mádʰu maniṣye mádʰu janiṣye mádʰu vakṣyāmi mádʰu vadiṣyāmi mádʰumatīṃ devébʰyo vā́cam udyāsaṁ śuśrūṣéṇyām manuṣyèbʰyas
tám mā devā́ avantu śobʰā́yai pitáro 'nu madantu ||
a The maker of the sound 'Him' is Vayu, the Prastotr is Agni, the Saman is Prajapati, the Udgatr is Brhaspati, the subordinate singers are the All-gods, the Pratihartrs are the Maruts, the finale is Indra; may these gods who support breath bestow breath upon me.
b All this the Adhvaryu, as he begins, begins for the Udgatrs; 'May these gods who support breath bestow breath upon me', he says; verily he bestows all this on himself.
c May Ida who summoneth the gods, Manu who leadeth the sacrifice,
d May Brhaspati recite the hymns and acclamations.
e The All-gods are reciters of the hymns.
f O earth mother, do not harm me.
g Of honey shall I think, honey shall I produce, honey shall I proclaim, honey shall I speak, may I utter speech full of honey for the gods, and acceptable to men.
h May the gods aid me to radiance, may the Pitrs rejoice in me.
As you can see here, this mantra is one of "[t]he Mantras for beginning the Stotra and the Pratigara". Let me explain.
As I discuss in this answer, Yagnas generally involved three priests, a Hotri who would utter Rig Veda mantras in their original form heard from the gods, an Advaryu who would utter Yajur Veda mantras while tending to the details of the Yagna, and an Udgatri who would sing songs from the Sama Veda. (Later a "brahmana" for the Atharvana Veda was added.) Now as described in this chapter of the Shatapatha Brahmana of the Yajur Veda, there was a certain part of the Soma Yagna in which the Udgatri would utter a hymn called a Stotra, and in response the Hotri would utter a hymn called a Shastra, and in response to that the Adhvaryu would utter a hymn called a Pratigara. This mantra was one of the mantras used to initiate that part of the Yagna.
The use of part of the mantra is also described in this excerpt from the Shankhyana Shrauta Sutras:
In this manner having summoned (the deities) he sits down with his knees raised and mutters, after having touched the earth with the span of his right hand: "May I not be separated from this firm standing, O Mother Earth. Hurt me not, scorch me not. I shall think what is sweet, wish what is sweet, engender what is sweet. I shall today utter words sweet to Gods, dear to men. Here do I by means of the fifteen-fold thunderbolt drive away my spiteful rival."
So to sum up, this mantra has no connection in the Vedas to the recital of the Rudram or Chamakam. People today may just use this as a Shanti mantra for the Chamakam because it's a mantra asking for blessings from all the gods. It would be similar to how the "gananam" mantra is often used nowadays as an Ahavaniya mantra in Yagnas, just because you're supposed to pray to Ganesha in the beginning of rituals. (Except the gananam mantra is not actually addressed to Ganesha, as I discuss here.)
