It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The one who gives the call to prayer (adhan) seeking reward is like a martyr soaked in his own blood until he finishes his adhan; every moist and dry thing bears witness for him; and when he dies, worms do not afflict him in his grave."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13497