Bhai Mani Singh Shahid records in Sikhan Di [[Bhagatmala]] of a story when Sikhs approached Guru Arjan asking for clarification on Guru Nanak, they said some people say Guru Nanak was an incarnation of Raja Janak. Guru Arjan replied:
ਗੁਰੂ ਨਾਨਕ ਜੀ ਪੂਰਨ ਬ੍ਰਹਮ ਰੂਪ ਹੈ ।
Guru Nanak Ji is the full form of Brahm (the formless divine )
ਜਿਸ ਦੀ ਬ੍ਰਹਮਾਦਿਕ ਸਿਵਾਦਿਕ ਜਨਕਾਦਿਕ ਸਨਕਾਦਿਕ ਉਸਤਤਿ ਕਰਦੇ ਹੈਨਿ । ਅਰੁ ਅੰਤ ਨਹੀਂ ਪਾਵਤੇ ।
Brahma etc, Shiva etc, Janak etc, Sanak etc, all praise that Divine, which is Guru Nanak, and are not able to find their limit.