It was during the time of Guru Arjan Dev Jee. Sangat was coming from Kabul (Afghanistan) to Amritsar for darshan of Gurujee. They met a Sikh and his wife on their way. This Sikh did a lot of sewa of the sangat. He massaged their legs, waved fan over them as they rested, brought water for them, everything. The next day they proceeded with this Sikh to Amritsar. As they reached Darbar Sahib, the jathedar of that Kabul sangat asked a few boys to take care of the shoes of everyone. None of the boys was ready to do it, as they all were very much eager to be the first ones to have darshan of Gurujee. At last this Sikh came forward and said I shall do it.
The sangat went inside and waited for 30-45 minutes, but Gurujee didn't show up. Then the jathedar went ahead and asked Baba Buddha Jee where Gurujee was.
Babajee : Gurujee has gone to see the sangat coming from Kabul.
Jathedar : But we are the sangat from Kabul!
Babajee : Didn't you meet Gurujee?
Jathedar : No Babajee, we didn't. But we met a Sikh and his wife and they did a lot of sewa.
Babajee : Where is that Sikh now?
Jathedar : We left him to look after the shoes of the sangat.
Babajee, followed by the jathedar, followed by the entire sangat went outside to look for that Sikh. They saw him cleaning all the shoes with his own chola.
It was none other than Guru Arjan Dev Jee Himself
Baba Buddha Jee went forward and took away the pair of shoes Gurujee was cleaning. Babajee said "Why are you doing this?". The entire sangat was now in tears. Gurujee smiled and said "Babajee, I haven't seen Guru Nanak Dev Jee, but you have. Take a look again at this pair of shoes". Babajee took a good look at the pair of shoes, and since he had been with Guru Nanak Dev Jee, he immediately recognized that it was Guru Nanak Dev Jee's pair of shoes.
Moral of the sakhi : When doing sewa at the Gurdwara, do it with this thought in mind that who knows who is visiting the Gurdwara today! Who knows which Gurmukh soul has come today! If you are cleaning a small kid's shoes, have in mind that it could belong to one of the Sahibzaade. If you are cleaning a bibi's shoes, have in mind that it could belong to Bibi Bhani. If you are cleaning a bhai ji's shoes, have in mind that it could belong to Bhai Bailo.