Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) said: My maternal aunt took me to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, my sister's son is ill. So the Messenger of Allah (peace and blessings be upon him) performed ablution, and I drank the water of his (peace and blessings be upon him) ablution. Then I stood behind his (peace and blessings be upon him) back, and I saw the Seal of Prophethood between his (peace and blessings be upon him) shoulders, like the button of a canopy (over a bed).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6650