Om! May God protect us both together;
May God nourish us both together;
May we work conjointly with great energy;
May our study be vigorous and effective,
and may we not mutually dispute (or may we not hate any);
Om! Let there be peace in me! Let there be peace in my environment! Let there be peace in the forces that act on me!

Oṃ saha nāv avatu
saha nau bhunaktu
saha vīryaṃ karavāvahai
tejasvi nāv adhītam astu
mā vidviṣāvahai |
Om śāntiḥ śāntiḥ śāntiḥ ||

ॐ सह नाववतु |

Om! That is infinite (Man), and this (universe) is infinite.
The infinite proceeds from the infinite.
(Then) taking the infinitude of the infinite (universe),
It remains as the infinite (Brahman) alone.
Om! Peace! Peace! Peace!!

ॐ पूर्णमदः पूर्णमिदम् पूर्णात् पूर्णमुदच्यते |
पूर्णस्य पूर्णमादाय पूर्णमेवावशिष्यते ||
ॐ शान्तिः शान्तिः शान्तिः || 

oṃ pūrṇam adaḥ pūrṇam idam pūrṇāt pūrṇam udacyate
pūrṇasya pūrṇam ādāya pūrṇam evāvaśiṣyate
oṃ śāntiḥ śāntiḥ śāntiḥ

The Shanti Mantras or "Peace Mantras" are prayers for Peace (Shanti) found in Upanishads. Generally, they are recited at the beginning and end of religious rituals and discourses.

Shanti Mantras are invoked at the beginning of some topics of Upanishads. They are supposed to calm the mind of the reciter and the environment around him/her. Reciting them is also believed to be removing any obstacles to the task being started.

Praśna Upanishad 2.13

प्राणस्येदं वशे सर्वं त्रिदिवे यत्‌ प्रतिष्ठितम्‌।
मातेव पुत्रान्‌ रक्षस्व श्रीश्च प्रज्ञां च विधेहि न इति ॥

Pranasyedam Vase Sarvam Tridivee Yath Prathishtitham|
Maateva Putraan Rakshasva Shrischa Pragnascha Videhinaam Ithi||

Meaning: Whatever exists in the three worlds is all under the control of Prāna. (O Prāna) protect us as a mother protects her sons, give us affluence and intelligence.