A famous saying by Guru Nanak. Something which reminds one of what Prophet Mohammed also famously said. This is the beginning of the Mul Mantra and is something that forms the very base of Sikhism.
ੴ ਸਤਿ ਨਾਮੁ ਕਰਤਾ ਪੁਰਖੁ ਨਿਰਭਉ ਨਿਰਵੈਰੁ ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੈਭੰ ਗੁਰ ਪ੍ਰਸਾਦਿ II
Ika ōaṅkāra sati nāmu karatā purakhu nirabha'u niravairu akāla mūrati ajūnī saibhaṃ gura prasādi
There is but one God. Truth by name, the creator, all-pervading spirit, without fear, without enmity. Whose existence is unaffected by time, who does not take birth, self-existent, who is to be realised through his grace.
There are various translations (all similar in meaning) and I have used one of those here for those of us who do not understand Gurumukhi. Listen to the Mul Mantar below, nothing touches you like the purity of this beautiful piece, especially early in the morning.
references: wikipedia, sikhiwiki