Mittar Pyaare nu, haal Mureedan da Kehna ...
I Dont think We can link it to Guru Arjan Dev ji's time because of the use of Modern Punjabi There......ਮਿੱਤਰ ਪਿਆਰੇ ਨੂੰ ਹਾਲ ਮੁਰੀਦਾਂ ਦਾ ਕਹਿਣਾ
according to Kahn singh Nabha in his mahan kosh. the above is written by guru arjun dev jee. it is one of the four letters written to his father the fourth guru from Laahore when he was sent to lahore to attend the wedding of his cousin. the other three letters are explained bya the kathakars but the fourth one they have attached to guru Gobind singh jee, because of ਯਾਰੜੇ ਦਾ ਮੈਨੂੰ ਸੱਥਰ ਚੰਗਾ .they thing he slept in the forest on grass and that is how they link it to guru Gobind singh.